As much as this would make sense to do, some bosses are very difficult if not impossible to do with certain group comps, Beauty for example would require two CC's for her pups if everyone was geared in 333 ilvl gear (although it is a bit unrealistic, understand that heroics are for the people who need the gear).
If your unlucky and dont have a group comp to do a boss, does that mean you should be penalized and prevented from finishing the dungeon? Im pretty sure thats the exact opposite of what Blizz wants