This is a creature that borders on average. As I've said before, 3 is the magic number when it comes to direct damage removal and there's a reason behind that: a lot of creatures have 3 or less toughness (shocking, right?). Why does that matter? Well, to start winning, you usually have to remove obstacles such as your opponents creatures, and the quicker you can get those chump blockers out of the way, the better, and since most chump blockers are tiny, you only need to deal 3 or less damage. Now, why does this matter here? Well, -X/-X kind of starts to turn around at about the X = 3 mark. Why? Because it can allow your chump blockers to take down your opponent's big things. If you're able to bring your opponent's big guys down to the size of your own little guys, then you don't really have to worry about direct damage in order to deal with them.
"But Jay," I hear masses cry out, "Direct damage works in the same fashion as -X/-X for the purposes of how you described!". Yeah, dealing a Timber Protector 3 damage with a Barbed Lightning will allow a Crypt Angel to kill it just as easily as if you hit it with a Last Gasp, but look at it this way: which of those scenarios results in you having a creature on the board? Hit the Protector with 3 damage and it still deals 4 damage, but give it -3/-3 and suddenly it only deals 2 which allows your Crypt Angel to live. Even if you can't kill it outright, I'd still prefer shrinking something down to block it instead of putting damage on it. And on the note of "not killing it outright", -X/-X gets around that pesky Indestructible thing, so that's a point in it's favour.
Back to this card. Yeah, it still has to tap and die to give the -3/-3, but at least it does more that Phyrexian Debaser at the same CMC. And besides, as I've stated, -3/-3 is a pretty good debuff to hand out, even if it's only once.
Pros: -3/-3 until end of turn
Cons: Has to tap, has to sack