I don't have many workhorses, just a couple yellow and red edged. But if I just pre-fold a flat/fst, it's just as easy to put on, right? I'm still on the fence if I want to keep the few fitteds I have for #2. We mostly just use prefolds as it is.
Depends on your diapering skills and how squirmy/kicky your child is. I haven't tried to put a pre-folded flat on DS in a while, since he squirms a lot and kicks at the diaper. It then becomes unfolded and I give up and reach for a pocket, AIO or fitted. I pretty much exclusively use my flats to stuff pockets now.
Re: FSTS/flats vs workhorses