Ive never had problems with using a Grass Type... probably because a lot of Grass Types are dual typed.
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Considering the type only has fourteen fully evolved non-legendaries to begin with, I'd say six pure defensive 'mons and a couple of tanks is enough to call it balanced.
Given the two types' moves, I would say that Grass and Poison are more annoyer types than defensive types. I'd say that Steel is pretty defensive because of its type resistances.
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I don't think Grass is the worst type to be honest. If you look at common elements, it does resist water and electric moves. And most are dual types, so they get the added benefits (negatives) of another type. Poison is probably a type I would consider worse than Grass. As for the starters, I actually think they made their Grass types quite appealing.
Grass isn't bad. It just needs help, either through dual typing or support pokemon. I'm not a big fan of them myself cause I like my pokemon to be straightforward battlers, and grass pokemon usually aren't.
a few that I do like:
It all matters on how you use it.
Gamefreak needs to look at the type matchup list again and make some more sensible decisions about what types are weak to poison.