I can understand if the reason it was left out was because it would clutter message boxes but why not just give users the option for turning it on, or better yet, give some kind of roll over option where if you highlight the text it tells you the time. You could even have it so you have to hold a certain key in order for the last point to work.
It seems evident that BNET 2.0 is being presented as basically an MSN messanger or AIM for gamers, so why not give it some more of the useful features of these programs.