A little late to this party, but I'm of the other mind, at least a little.
First off, I'm not sure the definition of a smart trainer, I assume it means one where the app. such as Zwift, can control the power. Which I have.
Of course a smart trainer gives you ERG mode and that is especially useful for things like fitness tests, or very structured workouts. But, otherwise apps like Zwift are doing everything based on power. So, it seems to me all that having the app change the power does during free rides, is force you to change the gears on the bike. With a dumb trainer of course you change gearing, but just to have the cadence you want at the power level you want.
And now, Zwift has the virtual gearing with some trainers, so you don't even change mechanical gearing, it just makes it harder if you hit a climb, then you "down-shift" by pressing a button, which just makes it easier again. What's the point of that?
Riding in the real world, has definite changes in pedal feel due to momentum effects. So, while average power my be the same, climbing still feels different from riding on the flat. Trainers, of any type, don't seem to do that, or don't do it to enough of a degree that I can feel it. So, again, what do smart trainers bring to the table, other than ERG mode? And no question, ERG mode can be a big deal for certain things.