Incorporating Robert Smith into your song prominently (like Blink 182 did with All of This)isn't a bad plan. I'm sure Billy just didn't wanted that distorted ethereal quality on just the chorus and didn't want Smith to dominate the track or even really share it. It WAS Billy's project after all. So no sweat. It is still an extremely beautiful track.
Eh. Robert Smith is icing on the cake for this track. I LOVE The Cure and I LOVE The Smashing Pumpkins. However, Robert Smith's vocals are so damn distored (and only come in during the chorus) that most people probably wouldn't even know it IS Robert Smith until they read some article or linear notes. In some ways I wish they had done (and released) an alternate take with Robert singing a verse.