Bachmann or a generic candidate emphasizing social issues would certainly be a weaker nominee than Romney or a generic candidate emphasizing economic issues. However, if one's goal is to advance a particular social agenda, then it would make perfect sense to vote for Bachmann. This is particularly true since even a weak nominee would have a decent shot at the presidency in this cycle.
I agree that Bachmann is clueless about many things, but not necessarily about this. Emphasizing social issues is indicative more of a strong social agenda than of a lack of strategic thinking (it's also evidence of political self-interest, since Bachmann is of course making a case for herself). Regardless, I won't be supporting any Republican for the presidency in 2012.