I've found gelatin to work almost as well at room temp as it does when the beer is cold. There may still be a little bit of chill haze, but it's much reduced over not using any gelatin. And it's much easier than chilling a carboy. I like to gelatin before racking to a keg since I don't want whatever settles out sitting in the bottom of the keg. Sure, the beer clears after a couple pints but the first time you move the keg it kicks back up again. You could always add gelatin to the keg, pull a few pints until the beer clears, then move it to a new keg, but again, it's much more work than just adding some gelatin to secondary.