If – on a mountain bike – you’re getting anything more from SPDs than a few % pedalling efficiency then you’re doing it wrong.
I don’t think that’s true tbh, there’s definitely other advantages- ask a pro downhiller, spds are better for control in general.
But, hmm, how to phrase it. If you need SPDs because skills deficiencies that stop you from using flats- ie “my feet come off” or “I can’t bunnyhop with flats”, that sort of thing- then those issues don’t go away just because you’re using SPDs- you’re really just papering over the cracks. But it’s almost certain that you’re still getting ill effects, because you’re not moving well with the bike, you’re working against it.
So, even if you stick with SPDs it’s really useful to have the skills needed for flats. Loads of people on SPDs already have these, they’re not flat specific, the only difference is that you can get away without them on SPDs- you get killed if you don’t have them on flats.
So if you think “I can’t ride on flats”, you’d probably benefit from learning, and then even if you prefer SPDs afterwards transferring those skills back to your spds