I dunno. Spike went 'good' before he ever had a soul. In fact, I found myself getting a bit annoyed with how the show set vampires up to be evil, then started backtracking and showing them as more sympathetic, less menacing (Harmony, anyone?).
I don't mind that a very few vampires either find redemption or at the very least aren't all THAT bad...but I think it undermines the whole premise if that happens a lot. It needs to be established that those incidents are very rare, and often ill-fated.
The Spike plot was something of a mess, largely because I think the writers were flailing for ways to keep the character around and only partially succeeded in maintaining his central appeal in the process.
I think Harmony worked better for me because she answered a question I'd always wondered: where the hell does every vampire get super ninja training? I was amused that she acted good out of a survival need (the demon community would certainly have no problem literally eating her alive, after all) rather than any real moral compass.
The in-game question, I suppose, is: would having demon-y PC's undermine things, as well? I don't *think* so, but I'm sure I could be convinced otherwise. In general, I somehow always got the impression that vampires were the Buffyverse pirhanna (eat anything and everything that gets within bite range), while other demons were, say, various shark species: dangerous, but not necessarily always, to the same degree, and/or to humans specifially. I suppose flavor-wise, though, we'd want to figure that out early-ish.