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Make it super effective agaisnt water or fighting.
More than an uselfulness problem is a team fitting problem. With the fire pokemon you have already covered the anti-grass quote, since is also se agaisnt 3 or 4 more types. There is no point for the poison one.
Although, some direct damage moves wouldn't be bad either, because having a pokemon just to cause poison then that's all is also a bit pointless, because even if you are the defensive type and use defensive moves your pokemon will probably fall long before poison has taken half of enemy's bar.
