When in doubt, I look to our ancestors for inspiration. They seemed to live a fairly good diet (when they could.) Our over-reliance on grains in modern days does seem to be a disastrous thing. In nature, grass seeds are a tiny part of our environment.
I cannot really speak about that but in most Asian cultures, rice( which is a form of grass) is a staple food. Yes. When outsiders consume them regularly they tend to fatten them up but I think it all depends on a person's individual needs......