Well, this is only my personal opinion, but motion capture is not animation. To me animation is an art. It's putting a little bit of yourself into the character. With motion capture, you're capturing a live action performance. And, I think, the difference in the quality of motion between a live action performance and good key frame is huge.
Motion capture has a very realistic look.
For example, if someone is clapping their hands, motion capture will give you a very accurate recreation of someone clapping. But, when you actually look at it, I think it would feel kind of stiff and lifeless, even though it's an exact replica of how a human claps. I think that to really sell the idea of someone clapping, you want to exaggerate it a little bit. You want to dilate the motion, so you really feel the contact of the hands slapping together. Whenever the characters walked along Andy's bed, someone had to go in and make bed dents, creating dents in Andy's bed.