I guess it would be better than not going at all, but I wouldn't do it, even though I love Universal. Even if you arrive on time, a half day isn't long enough to do even one of the parks justice, much less two (if you wanted to ride Hogwart's Express), IMHO.
If you DO decide you want to visit, I would advise not buying tickets ahead of time. You never know whether a flight will be delayed. We flew to CA a couple years ago to visit DL and DCA. Our morning nonstop, which was supposed to land at LAX at about 10:00 am local time, was cancelled due to mechanical problems. We ended up with hours wasted in two airports, and an arrival at the Grand a half hour before the parks closed that night. We had bought military passes that included that day, so we went ahead and popped into DCA for the last half hour of the day. The CM at the gate was surprised, but when we explained that we didn't have another day to go to the parks, she understood.
Instead, since it seems you are spending the rest of the trip at Disney, I'd wait, and if the flight lands early enough, add a day (which will be quite cheap) to your Disney passes.