>But you can't actually play emotionally expressive piano music on them -- not in the style of classical or jazz.Yes you can, and yes it is more-or-less trivial. With a small amount of deliberate practice, you can learn to produce a full and finely-gradated dynamic range on either a piano-weighted or a semi-weighted keybed. It's a one-dimensional mechanical skill that just isn't particularly difficult relative to, say, an oboeist's reed control or a violinist managing the very complex relationship between bow and string. If I say "you just don't press quite as hard" it'll sound like I'm being glib, but that's literally all there is to it, because physics.
Strings players choose from a wide range of string tensions and woodwind players choose from a wide range of reed stiffnesses, based purely on their own personal preference. Some prefer something soft and pliable that responds to the lightest touch, others prefer something that fights back when you dig in. It's an accident of history that mechanical pianos fall into a relatively narrow range of weightings, not a deliberate choice on the part of pianists or piano makers - the range of options are limited by the mechanics of an escapement and hammer.
Pianists overwhelmingly prefer weighted keybeds out of habit, but more generalist keyboard players will often prefer a semi-weighted keybed for versatility. You can play gigs or sessions with either and you're the only one who's going to notice. Calling either choice wrong is just dogma.