Saccing it right after the guy just unghosted is a move designed just to penalize him an extra 1/3 point of CON.
Even if we ignore that move, having destruction of an unholy weapon harm the AP is still useful. For instance, you might kill him, take the biggish unholy, then just keep it around for a while, until he gets some more pimp gear. Then you attack him, let him get semi-hurt, then flee/drop/sac to finish him. The risk, of course, is that he kills *you* and gets the unholy back.
Having the unholy harm the guy is also meaningful in cases where he somehow gets disarmed prior to dying. Sac weapon = (hopefully) dead AP. Point being: you're killing him once, just like you treat anyone else, because he's your enemy and hopefully you want something he has. You're not just giving him 2 deaths to be an ass.
