Well, you do get the grappling, tripping, bull rushing, etc. bonuses for Large size, the Stability bonus for having four legs, and the higher movement speed than normal. While it probably doesn't quite make up for the -1 to attacks and AC, -4 Hide, and more expensive armor, the other racial traits already make them slightly better than half-orcs. Overall, it may be just a tad better than a half-orc, but it's still enough that a half-orc PC would be rather pointless.
And anyway, as I said, I'd still rather see such races treated as powerfully as they're supposed to be, rather than beaten with the nerf bat until they've no real advantage over the smaller humans. Anyone who's as big as a horse or bigger ought to be stronger and tougher than a human, hands down. Centaurs ought to have more strength and constitution, like the ones in the Monster Manual and Savage Species.