I do admire the occasional patch in someone else's yard, on very rare occasions if it has stripes or a bright red color, but I would never devote space of my own to something called "ornamental grass" which sounds as oxymoronic to me as "ornamental weeds."
I admire the marketing genius of the person who started the phrase though, instead of us just calling it "grass that is too tall to mow." If my husband hears this phrase, he's going to want to know, what height does the lawn need to reach to call it ornamental?