Well, if you have your opponent shooting at the genestealers, then he's not shooting at your hive tyrant, or carnifexes. If he's not shooting at those, then they're going to get into melee with his stuff, which will win you the game. If they don't shoot at the genestealers, then your genestealers will get into melee.
So you have a distraction that is potentially destructive.
So you have a distraction that is potentially destructive.