To me, using the left hand thumb to play the lower strings of a chord really opened up the way I viewed and played rhythm. It frees up at least one finger to improvise and for me, it is also easier to play.
IMPORTANT: If your hands are too small to do this on an acoustic, it works just like this on electric guitars, and those usually have smaller necks! Try it out!
What do you think? Do you use your thumb in your rhythm or lead play?