It's not gonna take away alot.. Jesus you are overreacting. I've been to a lot of forums, and I know it doesn't take away activity from the rest of the forum. It'll probably not even be used very much.
Oh right.. I dunno...i think it will be more negative than positive but im not saying it will ruin the site. Of course it has its pros..it will be a great place for us to chat. However this site has to find a compromise of catering for existing members and making the place accessible for new members who may come here and be interested in what was said during a game.
In fact...i would change my vote to yes if it was open from 5 mins before kick off to 5 mins after the final whistle because lets be honest most of the stuff said then is small talk. However after a game we all like to analyse what happened and then its important that we have full posts for that.
I actually didn't have in mind for it to be a match-chat kind of thing, cos if I want to post about the game, I will write in an actual topic, not in the shoutbox. I still think it's a handy thing otherwise though.