Thumb on 6th string

I have noticed alot of pros using thier thumb to cover 6th string, kinda like what we use to call a "hillbilly" G chord. Any thoughts on the advantages or disadvantages to this method before I try to relearn some of my chords this way. Thx.
# 1

For some chords I use my thumb because it either results in a more comfortable grip, or it allows my other fingers to move around and gives me a better range. Hendrix was pretty big on it I guess, and a lot of jazz guys who do walking bass use it.
"When you're a young, long-haired guitarist, no one takes you seriously." - John Petrucci
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# 2
I can't! At least, I think I can't......
# 3

I tend to use standard barre chords, but will use my thumb sometimes. If I want to play chords without the E string, I can mute easily with my thumb. By the way, Hendrix had huge hands and a really long thumb. I think that his hands were a big reason that he could get some of his amazing sounds.
# 4