I don't see any harm on leaving in-active accounts on the site (Other then one of them might have a good "name" that another user might want - but if that is the case, i'm sure Fencer could fix that.)

Otherwise, if you start to delete account, you then have to check to see if they had played any games - make sure not to delete the games or the results for other players.

They take up very little disk space, and would probable (in my opinion) be more work to delete them, then there would be to just keep them there.