I'll see if I can dig up my baker's dozen potential downsides to carrying a pony bottle....
Oh, here they are!
1) Pony bottles add an extra piece of equipment to the diver's kit that needs to be fastidiously maintained to remain optimally functional.
2) Pony bottles add an extra training element as divers should be completely familiar with pony bottle carrying, deployment and usage as well as proper gas management.
3) Pony bottles need to be used frequently so that the diver remains proficient in deployment and use.
4) Pony bottles add an extra element of task loading in an emergency in that they require attention for deployment and usage.
5) Pony bottles if front mounted involve all of the above plus pose extra points of entaglement/snagging.
6) Pony bottles if back mounted pose extra points of entanglement/snagging that cannot always be undone successfully without help.
7) Pony bottles if back mounted valve up must be turned on prior to the dive and left on during the dive. It is possible for a leak or free flowing pony reg to empty or partially deplete the pony bottle.
8) Pony Bottles if back mounted do not allow confirmation of supply without the addition of an extra spg to the kit.
9) Pony bottles if back mounted valve down can be turned on when needed but the air supply cannot be confirmed unless an extra spg is added to the kit.
10) Pony bottles if relied upon as bailout can influence divers to make poor choices and extend dives beyond prudence or to even ignore gas planning altogether and simply rely upon the pony.
11) Pony bottles require additional set up steps that can provide opportunity for mistakes to be made... ie they complicate the rig.
12) Pony bottles can affect trim and require additional weighting adjustments, add drag and cause increased air consumption.
13) Pony bottles involve more cost not only for initial purchase but also for regulator maint. and tank vips/hydros.
