I think very many people would find it useful to have a 'message all opponents' button on the page. I've seen in a recent thread that a guy had to direct his opponents to his profile to clarify why he was resigning his games. I myself am caught up in a lot of crap in connection with a house move and would like to be able to explain to my opponents why I've suddenly drastically slowed down my response time to moves, but there are just too many to message individually. (Maybe I've done it now!)
This would be a wonderful idea for when someone has to go on vacation and would like to message all his opponents. I think the main objection to this idea is that any jerk could just message all his opponents with insults and what-not, but that could be instantly alerted and dealt with.
-Kev