If it works for the shooter, the caliber is really not an issue, and judgements should not be made by others. Personally, I wouldn't use anything smaller than a .20VT, .222 or .21 Fireball (I usually use a .22-250), but that's just me. I have been successful with these calibers (I usually shoot them at shorter ranges), although I could be equally successful with a larger centerfire for longer ranges. Who cares what caliber is used if it works and doesn't allow for the animal's suffering, or at least minimilizes the chances of suffering? If I were successful with a BB gun in 99% of my shots, I would not be too happy at getting criticized just because I used a BB gun.
Naturally, there is always the argument about "shot placement" as being an important factor. Well...Duh....naturally this is important, so why do people keep bringing it up? But, what do I know? I have seen people using 6mm bullets for ground squirrels. I kill them with a .22 LR and rarely miss....but I pick my shots and know exactly what the rifle will do. So....if a .17 HMR works, use it. Would I use a .17 HMR? No. I am not familiar with the cartridge and want a bit of extra ooomph in case my shot is not exactly where it should be.