Why does there have to be just one option at a given price point? Presently Apple does it but that doesn't have to be like that all the time. People's needs differ. And even more so, the needs are not a monotonous function of the money they are ready to pay for a machine. As Apple is growing, they can also afford to offer a greater variety of machines to match more people's needs.
And if someone want's an ultraportable laptop, the black mackbook is not really an option and the MBP even less.
There's more to it than simple consumer-choice. Let's say the following: if Apple were to produce 10+ options they would also risk diluting sales for some of those options. Behind these options is the production process, which needs to run to manufacture and market them. So, hypothetically, if Apple were to offer 10 options and produce 1000 laptops per option, and some options would go unsold, then Apple will have wasted money on producing these laptops.
Instead, what I see is that Apple is keeping a very tight production-routine. They offer options which really fit smack into the middle of a specific market. The macbook is for students and people on a budget mostly. The 15" is for professionals on a budget. And the 17" is for professionals that want the best a laptop has to offer. And that way, they can maximise sales per option. I'm generalising here.
Of course it's a little more complex than that. The non-pro Macbook-line is in fact quite spread out. It offers three models, which take up a lot of space and sort of close the budget + portable markets. If Apple were to offer a more portable version for a few 100 bucks more, the
marginal value would be quite low for people who would be fairly happy with a slightly less portable model at a few 100 bucks less. And other people would prefer a model with more oomph at a few 100 bucks more. It would leave the ultraportable to wither and die, even though a select few (but not the masses) would love to have one.
In other words, either the black Macbook would need to go or the basic MBP model. If you look at the previous setup of powerbooks vs. ibook, the situation was similar, but there was no black iBook, just a $1500 12" powerbook. And since both the Macbooks and the MBPs just got updated, I don't see a new portable in the near future. Sorry.
Incidentally, the same reasoning applies to a screenless desktop Mac.