The evil grass, is probably for ambience, for fun. It could have game play impact, but nothing can be said to that affect right now.
The different kinds of grass, is for internal logic and for Toady end gaol of making a world simulator. DF uses real life for its base line and inspiration. As we go back to the Panda example, it would be weird for them to graze on everyday yard grass when their diet consist of bamboo and metal spoons.
So yea, you have everyday grass then specialize grass such as bamboo and the underground flora. That would be at least 3 different grasses, and these category could use more types for variety.
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They may have different gameplay affects right now, but that remains unknown.
One player wants to plant sod (I think thats the right word) on top of dormitories for hobbit holes. I spoke about thatching.
I recall Toady speaking to the eventual farming overhaul with plants requiring different requirements to grow well, such as soil nutrition and watering. Though nothing was spoken to about nutrient content about the plants, it may come into play and these expansive grasses could have it, affecting herd sizes over time. (Though probably only noticeably in world gen, if anywhere.)