Why Analogies?

Rob's question is exactly right.  As a descriptive matter, the tendency to reach for analogies, I suspect, frequently derives from the fact that abortion analogies are used as cheap rhetorical cudgels with which to beat one's opponents into submission.  (This is, obviously, not true of everyone who uses analogies, but it is quite common to use them in this way and for this reason.) "Abortion is like slavery," so if you equivocate on its legal status, you are like the weak-kneed politicians of the 19th century who opposed slavery but refused to press forward towards abolition out of fear of the social unrest it might generate.  "Abortion is a holocaust," so if you think other issues are weighty enough to justify voting for a pro-choice candidate, then you are like those who sat by silently while the Jews went to the gas chambers.  And on and on...  If, however, abortion is sui generis, as Rob (correctly, in my view) suggests, then it becomes much harder to reject as out of bounds (or per se unreasonable) certain conclusions concerning how to weigh it against other issues.  And, of course, since talk is cheap, using these analogies imposes no costs on the person wielding them to act as we think those people who lived through the Civil War and the holocuast ought to have acted.  If you doubt my reading of the motives for using analogies in this area, I invite you to take a look at some of the comments in the story to which Michael P. linked below about Leslie Tentler's discussion of the recent election.