To each, his own. The Lee FCDs don't cover my mistakes, they are part of my load devlopment and normal loading processes.
As HSMITH very correctly points out, the "best" way to use the roll crimp shoulder in a standard seating die is to make it a two-step process. But that works well only if the cases are in fact the same length, some short cases may not be crimped at all and longer cases may actually bulge the case due to neck set-back. The only way to get consistant crimps with them is to trim cases regularly. The Lee FCDs, both types, are much less dependant on case length and I love 'em for that.
Jacketed revolver rounds usually don't "need" crimping unless the bullets are heavy (for caliber) and the charges hot enough to pull the pullets during recoil. Those hot/heavy revolver loads absolutely need to be crimped.
Lead bullet loads, both rifle and pistol, need much more case mouth expansion/belling than "normal" so the soft bullets can enter without cutting lead. Then, even if the load isn't hot enough to require crimping, it is imperative that the extra case flare be reduced to at least normal case diameter to insure proper chambering. The FCDs excell at that task without being length critical.
Many people, me among them, find we get best accuracy with FCD crimps IF the loads are developed with them. Just crimping an old established load may show no change or it may actually shoot worse!