I can't help but feel Mr Seto has a point, it's true that at least half the ghosts we have seen aren't outright malevolent and don't attack people, but from what we've seen so far they're not good either.
In case everyone forgot once Miko mistook a ghost for a random boy, that ghost wasn't doing anything outright malevolent, but as soon as he realized Miko could see him he tried to attack her, I think it's safe to assume that vast majority of ghosts would do the same even if they don't seem malevolent.