Actually, I think that if done right, one could pull off a one-pony scene pretty well. I'm with you that the whole project shouldn't be limited to one pony, but having certain sections of the music revolving around a single pony (where it is appropriate to do so, of course) could have a good narrative effect.
For example, in the movie, most of the scenes in the tail end of Act I and the first half of Act II revolve around a central scene/theme/idea of Pink sitting in a chair, staring at the TV. It wouldn't be impossible to do something similar using the Pinkamina party from "Party of One". But again, this is all up to the pony doing the PMV for those parts, and I just wanted to throw that idea out there.
*deposits 2 cents*