well, it is said that it would help not having distentions or tendinitis, but hell, to hold my wrist for that it would have to be VERY strong
so for me they would only be decoration
for wiping with after you get jacked off in a porta-loo and then throwing at your best friends girlfriend and letting her wear it for the remainder of the festival...
I use wristbands when I play guitar. I only need one for my right arm though so I dont get my bridge full of sweat crud and it smeers the guitar so I have to rub it off.
i used to use one on stage in my last band, sometimes playing would actually make my wrist a little bit raw, kinda sucked... especially on a guitar like an RG, where the edges are sharp...