I think Subbie has put his finger on a key issue here. Evolution, theistic or some other variety, doesn't necessarily challenge the beliefs of the liberal believer. But for people who may have a lower than average tolerance for ambiguity, it is viewed as the start of a slippery slope, and thus has strategic importance. It's a bit like Churchill's speech to the House in 1940:
"...we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender..."
To the fundamentalist who has invested a special authority in the literal words (as opposed to the general ideas) of the Bible, and who sees human existence as a "great controversy" *, every inch of the doctrinal landscape must be contested. The words of Genesis are seen as contiguous territory with all the other words in the text.
* Thus revealing the denominational roots out of which I evolved.
Capt.