Safety. But that only really applies if you're on three or more wheels or you've got enough clipless pedal experience that you're a liability without them. Even on fixed or an ankle-eating recumbent, the safety advantage of foot retention is cancelled out if you're just going to have a clipless moment.
Other than that, it's mostly about being able to fudge around your biomechanical oddities with cleat adjustment, which is only beneficial if you have some.
I can't speak about trikes or recumbents but what is the safety argument for being clipped in on a DF / safety bicycle? Even on fixed, foot slipping off has never been a problem.
Fewer bashed shins and inadvertent loss of bike control. This is mostly an off-road thing, thobut, and plenty of off-road riders prefer flats for speed of putting your foot down. YMMV.
It's much more important on a cycle where you can run over your own ankle if your foot slips off the pedals.
(On a trike there are almost no downsides to foot retention, as you don't need to put a foot down to stop. Completely unskilled users can safely use SPDs on a trike.)
And probably much easier for an ingrained cleat user to adjust to not using cleats than for a non-user to adjust to them, no?
Oh, sure. Not that that's relevant to the OP. TBH the main issue with flats if you're out of practice is forgetting to reset the pedals and faffing around when you start off at traffic lights.
Also you can do biomechanical stuff without cleats: I have 4mm of plastic under the insole of my left shoe!
That won't keep your knee aligned just-so, which was the main benefit (apart from reduced shoe wear) I got from SPDs when I first used them. It doesn't seem to matter as much these days, as I have stronger quads, but I probably wouldn't have got there without them.
My preferred approach to correcting leg-length mismatch is crank length. Mainly because I want to be able to use clipless sandals for touring, which precludes orthotics or shimming the cleat.