"Erg mode" is pretty core to expectations for smart trainers now. It can also lead to a sort of "workout failure death spiral" -- you start to fail at your interval, and your pedal stroke slows down, so the resistance increases to meet your power target, and you slow down some more...
This can end up chasing you up the cassette if you don't get back on top of it, or you don't adjust the workout bias (ie, reduce the overall power targets to something you can hit).
but you really want this, at least for structured training. It may NOT be important if your goal is "spin at home."
(Someone downtopic seems to think drawing a distinction between "people who mostly ride outside fast in pacelines" and "people who mostly just like spin classes" is somehow snobbery, but I don't see it that way at all.)
