There is a lot off difference between 'leaving 6 feet between' and 
'slamming into' the horse in front; there is a lot of difference between 
being slammed into at the water trough and only watering the horses one 
at a time.  And there is a lot of difference between a horse that kicks 
because the horse behind it bit it in the butt and one that is prone to 
take pot shots at anything that comes within striking distance.
It has been my experience, that most endurance riders will ride nose to 
tail, without any physical contact, and will pass (as the trail allows 
it) by moving about 1-2 feet to the side and going by (again without any 
physical contact--but not always, only if there is room). It is my 
understanding, in fact, that because riders are aware of the practice of 
riding nose to tail, if there is something in the terrain of the trail 
(e.g. a bunch of rocks, or some other rough going) which will cause the 
rider in front to slow down, he/she will raise his/her hand before 
putting on the brakes.  This keeps people from slamming into your back 
end (and is only polite--after all, the person behind cannot see the 
trail in front of you).
At every endurance ride I have ever been to, if there is room for more 
than one horse at the water trough, horses will share the water and 
drink at the same time.  (Yes this is difficult for the rider who has a 
horse that prefers visiting with the other horses to drinking, but...too 
bad.  It is unreasonable to expect everybody else to let you have the 
water trough to yourself, just because your horse cannot concentrate on 
the business at hand.)  This is different from a watering hole where the 
terrain dictates that it is only safe for one horse at a time.  In such 
a situation, of course, we should take turns (generally speaking, I will 
skip such watering holes entirely--depending on when the next water is 
available).
If your horse has been known to strike out in such situations (i.e. when 
a horse is behind it with 6 in. to 1 ft. to its tail, when another 
horse goes by within a foot or two, when sharing a watering hole/trough, 
or, I might add, at a 'controlled' start), it is a kicker.  You should 
put a red ribbon it its tail to warn people that this is the case, AND 
(I contend) when normal endurance riders would reasonably come within 
striking distance, pull off the side of the trail and let them go by 
(with the caveat, of course, that there is room at the side of the trail 
to do this safely--if not, they will have to wait until there is).
Yes, this gives an advantage to the person who has a horse that does not 
kick.  I don't have a problem with this; as I said, a truly good 
endurance horse is NOT a kicker no matter how fast, fit, or eager it is.
If you don't want to yield to all non-kickers on the trail, get a horse
that doesn't kick (or teach the one you have not to). Endurance 
horses should be able to handle close quarters (I am willing to draw the
line at actual physical contact) without striking out with their feet
(or teeth for that matter).  Unless and until a horse can do this, its rider must yield the right of way to horses that can.
Expecting people with kickers to yield the right of way and to ride in 
the back is not the same as expecting people who ride stallions or mares 
in season to ride at the back, as one is a matter of gender (which may 
or may not affect the way the horse acts), and the other is a behavior 
problem.  If, indeed, you have a stallion or a mare in season that 
bites or kicks, that horse should be at the back too--not because it is
a mare in season or a stallion, but because it does not know how to 
behave in company.
When I am riding a horse that kicks (and I have one...mostly I leave her 
at home, but when I don't), I do everything I can to ensure that someone 
does not suffer injury from my ill-mannered horse.  When she is in 
company, I watch the people around me closely, will verbally warn them 
(more than once if necessary) if I feel that they are beginning to 
encroach on her space, but more importantly, I try to keep her out of 
everybody's way.  Generally speaking, that means riding at the back 
(which she hates but I make her do it anyway).  I know the situations in 
which she is prone to kick, and _I_ keep her out of those situations; I 
don't expect everybody else to do that for me.
I feel it is part of my responsibility (since I am riding a dangerous 
horse--and make NO mistake, a horse that kicks is a dangerous horse) to 
be sufficiently aware of my environment that I am able to do this.  I do 
not consider it the responsibility of every other rider to keep an eye 
on me to make sure they don't get too close.  
Additionally, when possible, I will tell people in advance that I will 
be riding her, so that they may register their preferences.  I will even 
go so far, if invited to a group trail ride that someone else is putting 
on, as to ask 'is it okay if I bring Sonny?'  If they say 'no,' I don't, 
and I try not to bring her to any ride where I know there will be 
novices or children.  (Don't get me wrong, this is not a vicious horse 
that kicks at everything that gets close; I just don't entirely trust 
her not to kick if someone is riding behind (or beside) her.  Kicking is 
her first line of defense, and she gives no warning before she does it.)
Since an endurance ride is a place where I am almost guaranteed to find 
novices and children, I do not consider her to be an endurance prospect, 
and if I took her to an endurance ride, I would keep her well out of 
everybody's way--which means I would probably be at the back, and she 
would hate it.  But better for her to hate it than for someone to end up 
with a broken knee, rib, or nose (most likely nose as when she kicks she 
doesn't do it by half measures).  She is a hell of a jumper, and since 
jumpers don't have to share the show ring with anybody the fact that she 
can't behave herself in company doesn't disqualify her as a jumper.  It 
does disqualify her as an endurance horse.
When I am at an endurance ride, I consider it my responsibility to be 
aware of what is going on around me on the trail, and quite honestly, 
horses make too much noise to sneak up from behind.  I cannot imagine 
not being aware that someone was coming up behind me before they 
'slammed into' me.  When we ride, we give direction to our horses, we 
are not just along for the ride.  Maybe I feel this responsibility more 
than most people since I ride a stallion (and California law tells me 
that I am responsible), and it would never occur to me not to pay 
attention while I was riding.  But stallions aren't the only dangerous 
horses out there.  
If I am willing to concede that it is my responsiblity to control my 
stallion, no matter what his 'natural' tendencies may be, why shouldn't
riders of kickers be willing to concede that it is their responsibility to
make sure that their horse doesn't kick someone, no matter what their
natural tendencies.  If that means leaving the horse at home or riding in 
the back...well...nobody would hesitate to tell me that I should leave my 
stallion at home or ride in the back, were I unable to keep him from hurting 
someone (as well I should!)
kat
Orange County, CA
p.s. I do not consider a horse a kicker if it kicks at the horse behind 
it that just bit it in the butt; however, if my horse were to kick in 
such a situation, I would severely reprimand him; I don't want my horse 
kicking no matter what the provocation--just as I don't want him 
breeding a mare just because she's teasing him.  There are far too many 
teases out there.  
Just as I consider it my responsibility to make sure that my stallion 
doesn't breed a mare that the owner doesn't want bred; I consider it my 
responsibility to make sure that my horse does not kick a horse (or 
rider) that the owner doesn't want kicked (which I assume to be all of 
them--I can't imagine anyone wanting to be kicked).  It was easier for 
me to teach my horse 'never kick' than it was to teach him 'never breed, 
except when I say it is okay.'  He has, however, learned both lessons.  
So I feel confident in his behavior when he is out in a group.  He is 
what I consider to be a trained horse.  My mare, on the other hand, is 
only half-trained (well, maybe not even that much).  Maybe she will 
learn, but I am not holding my breath.