I have the same problem with my pack hitting the cantle (it's actually rubbed
the sheepskin off the top). But for the same reason you mentioned (getting
separated from your horse) I will continue to carry them. If I need extra
bottles I use a hornless pommel bag which has 2 carriers for bottles and also
2 zippered pockets (from Griffins) that could easily hold 1 each.
My riding buddy ordered a cantle bag that has bottle holders (will be
interesting to see how it works). Regarding the CamelBack, we've talked to
mountain-bikers on the trail and they whole-heartedly give them the thumbs-up
and say they are worth their weight.
Mary Thompson-Moore & Monte
Sunnyvale, CA