It was mandatory for use in Naxxramas. :P
You DO get control of the mob's abilities, look for additional bars above your normal ones. If you try to use any of your abilities the mob will break. Mobs of higher level than you will break more readily, especially if they have some sort of shadow resistance.
In general, it's mostly for use in trickery. On quests where there's an elite with guards, I MC the elite, beat down his guards with him, then release and zap him for an easy win. And, as mentioned before, it can yield good DPS/CC. Botanica, in particular, can be DESTROYED with tactical use of Mind Control. Back when I had a more even-handed spec I could play as DPS in Botanica and Shad Labs due to blatant abuse of Mind Control (now I'm always the healer). Mobs with area attacks are the best; you can usually do tons of damage to other mobs in the pull and get your MC target killed in the bargain.
A few tips for doing this. 1) Use Mind Soothe to get close, as MC's range is pitiful. 2) Keep the mob relatively close to you to limit breakage. If it breaks, shield and reestablish; don't wait for the tank to get it off of you, MC generates so much aggro he might not be able to.
In PvP you can't use the target's abilities; it's purely for affecting your target's position. The old stand-by, the infamous use of it, was in Blackrock Mountain, where people got tossed into the lava *all the time*. Other possible uses of it:
1) Arathi Basin. Toss someone off the Lumber Mill hill, into the water around the Blacksmith, or down into the gold mine.
2) Alterac Valley. Chuck someone off a tower or bunker to give yourself time to cap it. Very frustrating to your enemies!
3) The bridge in the BEM arena. Tossing someone off is tantamount to taking them out of the battle, at least for ten seconds or so.
4) Anywhere, really, where there's a cliff or bridge.
5) And, for the really malicious... challenge someone to a duel on the docks when the boat gets close, toss them off the pier, and watch them both forfeit the duel as they swim back and miss the boat to boot.