Here's a video demonstrating the benefits of not rapping your thumbs around the bar while benching. It makes it much easier to bounce the bar of your chest. http://www.youtube.com/watch?v=cEcv0Yn8z_c
Yes, seen this one before. A few more inches towards the neck and it would have been lights out. It also shows the benefit of taking a really deep breath before lifting the bar from the rack! This should be required viewing for anyone who thinks it's OK to bench with a thumbless grip.
Well... if you ARE going to bench with a thumbless grip, please do it in a rack with the safety bars high enough that you won't die if it rolls out of your palms.
Thumbless grip seems very foolish to me. I imagine the athlete compensates the thumbless grip by moving their wrists inward causing an unwarranted tension in the hands. More importantly, that compensation moves the load away from being directly over your arms and more towards one's shoulders.