[quote=@Lugia]
I can kinda see where you're coming from with the whole 'no damage' thing, 'cause in the PMD games moves that would normally do no damage (EX: Hyper Beam on a Gengar) instead do 1/8th of their normal damage. I... for some reason, prefer immunity. Maybe it's because I'm still mad at the anime for ignoring similar type matchups just so pikachu can win, or maybe it's just because I prefer using the main series games as a resource for this.
[/quote]

I'm pretty sure that's for balance reasons: Your max team limit in PMD games is only four pokemon, and that can be decreased even further by having a larger pokemon on that team or going into some of the more difficult dungeons. In many situations, you won't have the variety to deal with every type, so they give you a way of dealing damage that could be at least slightly better than the default attack.