It's based on the track record, as of late, of both companies. Pixar, has released consitently good material since their inception. Now, could they release a bad film? Sure, but it's obvious due to their autonomy from Disney (Disney only releases their films and have very little to do with their...
Pixar is its own production company with complete control. Disney just distributed the films. Pixar movies won't suck without Disney. But Disney, being in a rut, will suck without Pixar unless they can pull out of the rut before the contract expires.