If you have large enough hands, thumbing the bass string(s) is, for some, a comfortable alternative to making a full barre with the index finger. It ain't "proper" but it's "good enuff fer folk music". It works with some chord shapes but not others. It's easy to play a common G fingered 355433 either way, but I doubt anyone can play a G7 fingered 353433 any way but with a full index finger barre.

Personally, I sorta let the chord that's coming next dictate how I'll play a certain chord. If I'm doing a progression like E9, A, D9, G (like in Sweet Georgia Brown) I'll thumb the A & G chords because I use my thumb to mute the 6th string on 9th chords anyway. It also sometimes depends on the guitar I'm using. I may play a "proper" barre chord on an electric guitar with easy action, but use my thumb on a stiffer acoustic.      

And, I agree with Les B. Absolute clarity is not all that important for rhythm work. It's when the fingerpicks come out that the dud strings start showing themselves.

Bruce