pros & cons for rye grass

I keep hearing different stories about letting horses eat rye grass. Some say it is okay. Others say no. I am at a total loss. Anyway, any advice concerning whether or not it is okay for horses to eat rye grass would be helpful.

The only grasses I have heard to avoid are some fecues (non-endophyte free) and that's only for pregnant mares.
I know its best to have a broad mix of grasses in a pasture rather than one type only. Personally I like a mix of warm and cool season grasses for a longer grazing season

Ours are on rye hay. It's uglier than other grass hay, but they love it! (and I remember a friend went to the UGA hay seminar several years ago...came away with the understanding that rye was good.