I don't even understand why you'd want to coast on a trainer to begin with. That's the main benefit and the reason that you can get high-quality work done in a time-effective way: you don't have to coast (down a hill, up to a stop sign, whatever).
Maybe I"m not getting it because I've only ever done Zwift on a dumb trainer and 90% of the time I ride Zwift it's to do a structured workout. I'm not sure I'd use Erg mode even if I had the option.