As has been mentioned at least once, the minibuffer history bindings are in minibuffer-local-map, not global keys, and would not work as global keys.

Chong Yidong's suggestion for binding in text-mode seems like a decent compromise, to which I would personally suggest adding outline-mode and all programming modes.  That said, I don't see any reason for the compromise, as the objections seem to be your confusion and the fear that it might become some sort of defacto standard -- whereas I am suggesting that the goal is exactly to make it a defacto standard, ala M-a, M-e, TAB, etc.

It would definitely be a change, although one most people are unlikely to notice (they either already have a personal binding that trumps it or they don't  have an expectation about the key being bound at all).  That's why I suggested it for Emacs 24.

I hope that helps,

*Chad