Okay, I'll toss my thoughts in just to round out the mix...
I often use a pony bottle when I dive - even when I dive with others. For me, the pony is just another safety measure for me. I don't plan to use it and I don't dive in confined areas (mostly open water recreational dives). As such, I prefer mine attached to my main tank so it's out of the way. Yes, I cannot see it or "keep an eye on it" but I am rather confident that if I need it, it will be there and in a ready state. I check it before I don my rig and I often will check it early in the dive (just to make sure it's working).
There are many who will state all the "bad" things about having a pony on your back and I suspect for some (techies or wreck divers) this may be true. But for the recreational diver, I will argue that it's an easy bit of gear to add to your set-up that may prove to be really helpful someday.
I have a simple bracket that my upper tank strap slides through and sits with a low profile (if I am not using the pony - or it can be quickly removed). The pony itself has a band that holds the bottle (no hose clamp) and fits onto the bracket and is held by a pair of quick-release pins. The pins are sandwiched between the main tank and the pony. In theory, another diver could pull the pin and remove the bottle if need be. Although it is not intended to be for any quick exchanges, the bracket is called the Quick Draw.
http://www.quickdrawbracket.com/SETUPS.ivnu
The pony (19 c.f.) does sit off to one side. Toward the end of the dive, when my main tank gets low and the pony starts to become noticable off to the one side, I can then slide my weight belt around a little to compensate for its weight. It's no biggie (sometimes I don't even bother).
Again, in my use, the pony is not intended to extend my bottom times it's only a back-up. I use this when I solo dive and when dive with a buddy.