I use one (it's the older all-wood version, not the new sleek aluminum model) and probably use it less "as-intended", and more for troublesome necks. I find that maybe one in four necks call for it throughout a fret job, but it's great for getting a good "initial read" on neck flatness, since it consistently holds a neck without flexing. Any fingerboard leveling can be done at the same time, since the rigidity is so good.
The other features I like are (1). the ability to use a surrogate body (electrics, of course) giving unimpeded access to the upper frets without worrying about body damage... and (2). the option of tilting the guitar upright to the playing position when doing final "fussing" and action setup. I recall being surprised at how much of a difference that made, versus laying flat.
From what I understand, I'm probably not "using it right" or taking advantage of all it has to offer, but (as it's used, for me) it's a good tool and worth having. Not sure if it's 100% necessary in the long run, but it's been handy to have.
i don't do a proper fret leveling or refret without it!
mine is the old-school wood version and a copy at that, but it still serves to let me eliminate the errors that happen when the strings come off and the truss rod gets loosened (where a neck might have warps and bumps that "present" under load but disappear when all tension is taken off). yes it takes more time to jig up the guitar, but i can level or even refret without having to futz around spot-checking my work with the little fret rocker or whatever, i know the results will be predictable when the strings go back on.
guitar goes in the rack while strung and tuned, rack gets flipped sideways into playing position (very important, you can watch those dial indicators spin when you flip it up), neck gets adjusted to maximum straightness, locked down and strings removed.
another critical detail is the way tension gets simulated, by pushing up at the headstock and pulling down at the nut; this serves to reintroduce the upward twisting force of the string pull on the headstock, which can show up as humps and valleys all over the neck. if you don't account for this force you might end up with an annoying surprise after all your careful fretwork is done and the strings go back on.
i use it for leveling the board itself too; lock the guitar down, pull the frets, then level the wood while still in the rack. here's where i might introduce a compound progressive radius if appropriate.
assuming i do a good job with the new frets in sizing the fret slot tightness right, this gets me a board that itself is straight when strung up, leading to less fret leveling needed afterwards.
yes it's more work to set up, but again it eliminates any doubts in the result so i never have to waste time looking for errors or even doing things over.
One thing I liked the look of was watching Dan Erlewine accurately measure how a neck was reacting to compression fretting and how much easier it made the task.