I think it helps to keep in mind just how big TWD is right now. It's the #1 scripted show on TV. Not #1 genre show, not #1 cable show--#1 scripted show across all networks. Only football gets bigger ratings--not American Idol, not Big Bang Theory, not Game of Thrones.
There hasn't been a horror property this big in the mainstream for at least a generation (I'd say probably since The Exorcist). There probably won't be another one for another generation. And right now, the show is peaking--record ratings for its season finale. Universal already has the rights*, they'd be crazy not to use them. This is like being able to build a Star Wars ride in 1977, or at least an American Idol show in 2005 (not years later like DHS did, when the show was already in decline). The general public eats TWD up with a biscuit, if you can feed them more without having to commit to major park infrastructure, why wouldn't you?
By 2014 or 2015, TWD will be played out, like any hit show, and I doubt there will be another horror IP mainstream enough to take its place as the "face" of HHN. Until then, we at least have set SZs that should help manage the crowds a bit, and--by my count--6 promising houses, not to mention a Bill & Ted from the writer of last year's above-average show.
* Make no mistake, I think Universal got lucky here, licensing the property early on for just a house, not knowing it would explode like it has. But when you stumble upon a cash cow, you milk it.