You get yourself set up playing a few rounds in a good server with your buds, or you've found one with a lot of good team players and no lag, but you want to give your Engineer / Operative character a break and work on your Soldier / Medic character. Would be nice to be able to swap them over between matches without having to drop from the game and go all the way back to the menu, no?
Would be even better if you could do it before the match starts, but after the map is selected, so you could pick the body type you like for that map, but some people might have a problem with that.