I can't say that I'm in favor of having a sub now. However, let's say that we had a number of players who are on the fence about playing. They could sign up to take over in the case of someone having to leave or being inactive. Under no circumstances should a player ever get a second chance, such as in the case of Hardy taking over for Kat, however.
Kaoz is correct that hostkilling can radically change a game, but until we come up with a better method, it's what we currently do.
If there was someone who wasn't involved in the game at all and didn't frequent the area and was unaware of the roles or anything, I wouldn't really have a problem adding them in right now. 3c would be fine, but as we've already addressed, he's a no go. I have no idea where blai is. Evil's busy, you get the picture.
Let's say that we were to get a replacement for Kat right now. This conversation has told us that Kat was a townie, otherwise Hardy couldn't take over for her as he'd be switching sides, and that ruins some of the surprise.