My question is then, why throw all this money at something else, when you have the next Potter cash cow being built in the same park?
I can see more than a few reasons:
1) Right now, IoA is the superior park all around. Even with Potter 2.0 in USF, I'd probably give the edge to IoA over USF thanks to just superior place-setting. Comcast/Uni may know that as well and wants to level the playing field, so to speak. Not to mention IoA has the 1-2 punch of Spidey/Journey. USF will have Gringotts, but not another ride on that level.
2) Uni knows how insane WWoHP will be and wants another new-newish attraction open before/around the land's debut to try and redistribute crowds.
3) In the same vein as #2, if Potter isn't opening until, say 2015 due to a delay, or even just 2014, if they can say "Hey, don't delay a trip! We've got ______ opening in 2013 and it's gonna be awesome!" they can ensure people will continue to visit WWoHP 1.0 and the new ride.
4) 2014 will be USF's 25th Anniversary. I could see them wanting the park to blow people away that year. Rolling out Potter, a new version of Cinespec, an updated parade, and also hyping SS44 project would be highly effective.
5) If rumors of KidZone going down after Potter are true, the park will need all the people eaters it can get.
6) Because they can. Comcast wants to make these parks all they can be.