I suspect adjustables are basically an attempt to somewhat compensate for relatively worse performance of safety bar plates to cope with long hair growth compared to open comb plates, while allowing users to revert to a safer, if possibly somewhat less efficient setting for more frequent shaves.
Except the few I've tried still weren't as efficient on the first pass on their highest setting as an open comb.
My main beef with adjustables is that many of them choose a base design with poor blade support, so they're kind of unpleasant to use even on the lowest settings against the grain, at least for me.
They're also mechanically complicated and more difficult to take apart for cleaning, which I don't like in a tool that can, potentially, come in contact with blood.
There are some safety bar razors which have a tiny amount of blade gap while being fairly decent for mowing down moderate amounts of growth (about two weeks or so), and they don't need adjustability either. They do have excellent flow-through characteristics, though. Technique matters, too.
IMO, a well designed open comb razor doesn't need adjustability and will perform well as a daily shaver as well as on months of growth. I'd probably reach for a shavette or a straight for lots of growth, but then not everyone fancies using them - probably mostly people frequenting shaving fora.
At the same time, I don't see a problem recommending an adjustable to beginners. They'll try a high setting, bleed for it and revert to a lower setting until they get the hang of it, coming back to try a higher setting once in a while to gauge their technique.
For me, using shims has been a much more pleasant experience to adjust a razor than using an actual adjustable.
