Theres a couple reasons you might still want a metrocard. One is that OMNY restarts your 12 ride count every Monday, so if you're visiting from out of town you might be better off with a 7 day unlimited card (depending on which days of the week you'll be using it). They also haven't quite finished rolling OMNY out everywhere; I know the Roosevelt island tram didn't have it as of last month, and maybe there's some other niche services that haven't been updated yet.

Aside from that i really like OMNY, though. They seem to put a lot of thought into it.