High level players would execute multi-attack at the same time anyway. Even maphackers can see them coming doesn't mean they can defend it.
Same logic as CoH1 4 ways Rifles/flamers flanking
if i play against a braindead person, i will win no matter what.
if i see what my opponent does and i can't defend it, i shouldn't have been matched against it in the first place. of course, maphacks won't get a bronze player to grandmasters, but he WILL win more games than he would without.
also, there ARE maphacks in sc2. some users get banned, some don't. if you watch a replay and see somebody blind cannon rushing your starting position on a 4 player map wihtout any scouting... how do you think he is able to do that and still win more than half of his games?