hmm does grass or carpet are made of hairs ? lol it's not a tool for making actual hair, it's a shader to put on hairs or fur , it's different. If you'r capet is made of fur, like a bear carpet, then yes, if it's a fabric carpet, then you'r better with the coronaMtl .
Well if you look at a carpet close enough, you will most probably see that it's made of fibers which are pretty much hair-like (they scatter light, are reflective, etc), so if you need super high quality carpet for close ups, I think there is nothing wrong with using hair plugins with hair shader for that. But generally you can usually get away with scattering some strands and standard Corona material.
I wonder if there is any way to make a grass field with some hair plugin + hair shader. I can't really think of any benefits from doing that, but who knows... As far as I know Ornatrix can be used to scatter trees, grass, or other plants.
Just a note on using 3ds H&F modifier: just forget about the effects dialog. Leave those settings at defaults. Then plug Corona's hair mtl into the mr shader slot. That's the only workflow for now.