rcmodel (and walkalong) have the issues regarding the 4th / seat and crimp issues laid out nicely.
I've loaded 1000s of rounds on a (Lee) 3-die setup--both .45ACP and 10mm. Once I had my recipes sorted out (and that includes a specific bullet, as supplied by a given supplier), I could tweak up my (Lee) seater-crimper to do the appropriate taper.
However: when I got curious about load development again, as in "I wonder how WSF works compared to 231?" or "Will my SA 1911 with the BarSto barrel like 185LSWCs better than the H&G68200LSWCs?" then I would be in for extended tweaking. And that's without talking about the .451 / .452 issues, and LOA / feed questions that would seem to come up....
So, even for semiautos, I think a 4-die setup (separate seat and crimp) has a real place in load development. Today I just finished loading up 10 mm dev't boxes--Speer 180TMJFNs and WSF / Silhouette / #5 loads, to be used as burnishing loads in a EAA Elite Match 10mm, and to look for sweet spot recipes for that pistol and for a Kimber ST II. Those loads were built on a (Lee) turret, currently set up with an FCD in #4. I'll also be comparing / testing the FCD against the (standard Lee) 10 mm taper crimp die.
FWIW, the FCD die does have its place in 'cleaning up' poor ammo. The post-sizing works for me when I load some early 10mm 200-gr LSWCs that seat so deep as to make chambering an issue. (These bullets probably should just be put out for salvage, but I am too cheap--they cost me $27.00+shipping when I bought them about 15 years ago.)
Jim H.