Well, yea, he does. Doesn't he?
Someone already brought up the bamboo example. Then there is underground flora, which is mostly fantasy work their as well. So, that would be at least 3 (the third being everyday normal grass) different types of grass. And their could be a myriad of other grasses similar to the bamboo example, where it'd wouldnt make sense for a panda to be eating regular everyday grass.
So, even without much effort the number of grass types expands with itself. However, beyond actual grass types that possibly in the game, we're also getting some more fantasy flora, such as evil grass. The evil grass, is of course complete fluff (though it may have game play impact, such as environment troubles for the adventurer or fort), but fun none the less. I for one, do not object for fun things being added to a game.
With how Toady has set out for DF, I don't see how he could have avoided extra grass types.
And least we forgot, they may not have any gameplay difference now, they may/probably will in the future. The various trees types currently in the game are well, the same damn thing, buts known that they will eventually get expanded. When? Who in the fucking worlds knows. But they well.
It took a wii bit before the cheese and soap makers to actually be useful. They were in the game way before actually had a use.