Okay, attempting to jump the train back onto the tracks...
As has been said before, I'm also of the persuasion that the good girl / bad guy pairing-conversion is only plausible if the bad guy was on his way to switching sides already. One thing I have not seen discussed here, however, is the idea of a pairing resulting in a good person who ultimately becomes one of the villains. To me, that'd be a lot more interesting.
A very good example of why the bad-good conversion usually doesn't work is actually presented in the filler arc of timeskip Naruto involving the Sanbi, with the relationship between Guren and Yuukimaru. Yes, I know that's not romance, no I don't care - it still follows the pattern. What really got to me, though, was not only the incredibly short time it took for the conversion to happen at all, but that it was so complete that it effectively made a cold-blooded ruthless mass murderer into a tree-hugging pansy that was no longer capable of defending herself against her own minions. There is such a thing as going too far to make a point.
Overall the concept just doesn't appeal to me that much anyway. Besides the far fetched nature of the whole process, there's something else to consider - depending on the 'rank' of the villain being converted, a writer can easily destroy any antagonistic elements with which to continue his / her story simply by having no real material source left to work with, unless they decide to go with an OC. This is tantamount to writer suicide.