Reliability isn't there. They're not as efficient. They're not cheap to make, they're not mechanically simple. Making one pass emissions standards takes too much effort. They're compact but not by a whole lot, and their power/weight isn't significantly better or anything. They're a strange/interesting sort of engine, but I don't believe that justifies its continued use.Not a lot, really...
Mazda needs to bring back the turbo MZR and poke it in the Miata.