In addition then we would need "block PM" button to deny those who you don't want to send you PM,as it could be used to distract while noone else sees it. Otherwise it's not that bad idea,could be helpful to communicate with teammates in some leagues where public chat is forbidden.
+1 point for the idea. The private chat is available in Airio and LFSLapper but its too difficult to write every time /pm xyz and the message, because there is users who have "abcdefghijklmnoprstuvwxyz" long usernames with Ł$€#>># charachers. So a simple button with a small windows it would be very cool. I hope devs will think about it.
Its good idea, but bad thing about this is that it will "isolate" people.
For example two gays talking (lets asume they are using private chat), and third gay join server but he doesn't know there are already two gays, so he can't join discussion about gay stuff.