I disagree. That the theory is true doesn't mean that it does, can (or has to) explain everything. That I have short nails today, for instance, need not be reconciled with the theory of evolution. I just cut them yesterday. The proximal explanation suffices (of course it is possible that nail-cutting has a fitness advantage and has been selected as a trait ;-)). The "spandrel" explanation is not just a fallback (leave alone a desperation). It is rooted in physiological data and in that sense is more atomic and authoritative than adaptationist explanations, which are both higher-level and in need of external validation. The problem with adaptationism is that similar to religion it asks an elaborate question which it claims to hold the sole answer for. Fodor, whom we discussed recently, argues similarly (similar to me here, that is) against adaptationist triumphalism in the issue of the mind.
Try it out. In the absence of other evidence, which one sounds more parsimonious:
a) the male nipple and the female clitoris are where they are because they share a developmental path with a corresponding organ in the opposite gender which has clear functional advantage.
b) the male nipple exists because it provided a selection advantage for individuals possessing it, over non-nippled males, perhaps to keep baby occupied as the Bushmen do today.
My (b) is not analogous to or a caricature of your "clitoris as male quality selection mechanism" theory (which is coherent) but rather is an example of any version of anti-"spandrel" story that has to be offered about the male nipple.
--ravi