I don't have any better explanation that anyone else for the Miers nomination. In my too-clever-by-half moments I think that something like this is going on: that Miers is an intentionally disappointing nominee; that a republican strategist has no intention of seeing Roe overturning -- so that it can be continuously used in future elections to push the republican party even further to the right ('we need politicians who are even more socially conservative!').
It's also possible, I suppose, that the right really does want her, and is trying to dupe Dems into not opposing her. That this intra-republican party war a smoke screen.
But I think it's more likely that movement conservatism wants a fight while the White House wants someone to seem inoffensive. And that an unexpected consequence of the Miers nomination is that some on the right are realizing that Bush is actually not very competent, i.e., he doesn't do the job of being president very well. Some of them are just now realizing that his judgment is bad, his decisions arbitrary, his governing philosophy non-existent. Of course, it would have been nice if they had figured this out before they all went and voted for him last year, but one can you expect from the modern republican party?
The good news for Dems, I suppose, is that this nomination could be bad for the long-term health of the republican party. Really makes one wonder how a republican nominee in 2008 will handle the Bush legacy. Unless things turn around dramatically, it seems like Bush is headed for a unusually unsuccessful two-term presidency, unsatisfactory to the left and the right, and that the lack of success will be obvious all but the most blind partisans. How does a party run on such a legacy?