I can see the justification on paper for a larger-screened cousin for the 12" MacBook, much like how we had the 12" and 14" iBook.
The thing is, apart from a small minority, I don't see a large segment of Apple's userbase crying out for a larger screen consumer-grade MacBook. Indeed, most of the criticism levelled at Apple's MacBook lineup seems to be directed at the higher-end range, with the MacBook Pro and it's lackluster specs/lack of ports -- and if the rumors are true, Apple intends on addressing those complaints with the next MBP revision.
That's not to say that Apple would never release a 14" MacBook; they could probably do it. After all, we have seen Apple fielding a consumer-grade notebook with a bigger screen than a pro-grade notebook in the past (the 14" iBook vs. the 12" PowerBook). However Apple would first have to be sure that there would be a market large enough to make such a product viable. Otherwise, they won't bother.