Yes it has. Probably lots of times
But anyway...
Personally I'm pretty sympathetic to your argument about Birmingham, but not Milton Keynes. Using the reasoning in my other post from a few minutes ago, a through station at Birmingham, with all trains stopping there instead of terminating there would mean that most places would get far more frequent services to both Birmingham and London, using exactly the same paths. To my mind, that would have made HS2 even better (albeit a lot more expensive, which I'm sure is the main reason why that's not happening!)
But I wouldn't support running through Milton Keynes: I would say that the difference is that Birmingham is easily big enough to justify stopping most/all London-the North services there, with the slightly slower journey times more then compensated for by the frequency increase. But Milton Keynes - with its massively smaller population compared to Birmingham - is nowhere near big enough to justify stopping most HS2 services there. Plus Milton Keynes is a long way from the direct line between London and Birmingham that HS2 roughly follows, so you'd have to make HS2 a fair bit longer to serve Milton Keynes. And besides, Milton Keynes is already going to be one of the biggest beneficiaries of HS2, since basically everything on the WCML will be able to stop there post-HS2 (I would guess that, currently, it's only about half the trains stopping there, so MK will already see massive improvements in services).