The only niche market being served is the education market who will, ten times out of ten, go for whitey over the other two. To everyone else, you could easily pick between any of them; the only one of the two that is specifically for people that the other one isn't necessarily is the MacBook Air. Some need that portability, others would rather have a complete computer. Otherwise you could sell a white MacBook over a 13" Pro or vice versa to just about anyone. I've done it both ways to many people, none of them know the difference or care. To them, the fact that they're on a Mac is far more important than what color is that Mac or whether it's plastic or Aluminum. I think very few people actually pay attention to the material used. They might pay attention to the color differences between the two, but that's it.
As for the argument of "ZOMG, if they kill the 13" Pro, they'll lose so many customers!!!", first off, if they were to only drop the "Pro" name, but otherwise reintegrate it into the "MacBook" line, they'd lose 0 customers. Those three letters only mean something to those needing a machine to do tasks that can be described by those very same three letters. Secondly, if they discontinued the damn thing outright, if there were any customer loss, it'd be minimal, if any at all, especially if they were to reintroduce a black MacBook, as unless we're talking about people only buying a Mac because it's shiny, people will just default to the next lowest one or go higher to the 15" MacBook Pro. It's not like a majority of those customers would just disappear if that machine did so too. Some might, sure. Enough to make Apple go bankrupt? You must be joking.
This forum isn't representative of anything other than what happens when you cram a crap-ton of opinionated Mac fanatics into a forum and have them go back and forth on their opinions, predictions, and predictions which are subconciously heavily based on opinions. It's representative of nothing other than the refresh needs to happen so we can all stop talking about the MacBook Pro for a year and then (hopefully, more intelligently) discuss the future of the next Mac in line for a refresh, or, y'know, go on about how we like the "new" MacBook Pros. Either or.