Atygrit,
Welcome to reloading and thanks for asking our advice.
Lots of good advice, but some of it may appear contradictory. Here's mine and my reasoning behind it.
First a few facts that are not in dispute.
The Seating die has a crimping shoulder in it, so it CAN, but does not necessarily HAVE TO crimp and seat at the same time.
Prove it for yourself. Take the die apart, clean the inside and shine a flashlight into it and look in the bottom end of the die body. You will see the crimping shoulder.
Flaring the case mouth and crimping bends and stretches the brass. This is known in metallurgy as "cold working" and makes most metals brittle. We have all done this with a wire coat hangar. Bend it back and forth a dozen times and it eventually snaps. Flare and crimp a case mouth enough times and it will eventually crack (unless you soften the metal by annealing it). Use excessive flaring/belling and strong crimps and you speed up the process, shortening your brass' life.
Lee FCD (factory crimp dies) have a carbide sizing ring at their bottom. This re-sizes the case as it is withdrawn from the FCD and forces it to be cylindrical as the LAST act of reloading. GP100Man mentioned this in post 9, third paragraph) The reason for it is to ensure the shape of the cartridge case is restored to SAAMI dimensions if the case-mouth belling die or the crimping die caused the case to be re-shaped, slightly crumpled or over-expanded by a too-large bullet (which happened to me with some oversized cast lead bullets once).
Some reloaders don't like this sizing ring and actually knock it out of their FCD.
The reason for the existence of the FCD is that, when you seat and crimp with the same die, you are actually crimping WHILE THE BULLET IS STILL BEING PUSHED DEEPER INTO THE CASE. If you are applying a gentle crimp, this does not present a problem. If you are applying a strong crimp, this can actually cut into the bullet and shave lead. (see post #2 from kimbernut)
If you seat the bullet to the proper depth without crimping there is no lead shaving. Then, assuming the case mouth lines up with the crimping groove, if you, in a separate operation, apply crimp to the case mouth, it will settle into the crimping groove without shaving any lead.
You could actually do this without the FCD. Take the seat/crimp die and use it to seat the bullet without crimp. Then remove the bullet seating stem and use the seat/crimp die to apply a crimp WITHOUT MOVING THE BULLET DEEPER INTO THE CASE. But this is inconvenient for most folks and does not apply the final FCD sizing ring to ensure proper shape of the case as it is withdrawn from the FCD.
The two main purposes of the crimp applied to cases that headspace on the rim is 1) to prevent the bullet from being pulled out of the case by inertia as the gun recoils from its siblings being fired. Light guns and heavy loads are particularly prone to this, and the bullet does not have to completely exit the case to cause trouble. All it has to do is protrude from the front of the cylinder, thus jamming the gun. (I am sure this is why Marlin 45 Carbine, in post #3 said, "I don't use the Lee FCD on my 'plinking' loads." and Cheygriz mentioned "full power loads" in post #5) Happened to a fellow in Soldotna while he was being attacked by a bear. Fortunately, the first two shots disabled the bear, but he had to call in another firearm to actually administer the kill shot. The second reason is to ensure that (with certain powders that require a goodly amount of pressure to burn properly) the bullet stays put long enough to build proper pressure inside the case. By this time there is enough flame that the powder will reliably continue to burn properly. Erratic burning rates of slow powders (particularly problematic with light loads) has been known to blow up guns. (Do a search on the word "Ka-Boom" or the phrase "Do not reduce loads..." H-110 is notorious for this. While H-110 is an excellent choice for high-end magnum performance, light loads are dangerous. (see post #5 from cheygriz)
Now for my opinions and advice.
You have a 4-station press. If you are utilizing the auto-indexing feature, you don't save any strokes of the handle by skipping the FCD. Use the FCD.
Adjusting the dies separately for seating and flaring/belling is simpler than adjusting a single die for both. Not a big deal, but it does save some complication. Use the FCD.
If you are loading heavy loads, you will want a strong crimp and probably will shave lead if you combine the seat/crimp operations. Use the FCD.
If you are using the press like a single-stage and want to minimize the time you spend loading, skip the FCD. You will reduce the number of operations from 4 to 3.
Always wear safety glasses, especially when working with primers, and don't pinch your fingers in your press. Verify for yourself everything you learn from casual sources. Believe only half of what you see and one quarter of what you hear. That goes double for everything you find on the internet.
Happy New Year,
Lost Sheep
P.S.
By the way, "flair" means "a natural ability to do something (e.g. He has a flair for storytelling.)
"Flare" (aside from its primary definition, to burn unsteadily) means : to open or spread outward <the pants flare at the bottom>
both definitions copied from Merriam-Webster.com/dictionary, which also has included in the definition of "bell", this example, "d : the flared end of a wind instrument"
Thus, you can bell a case mouth and you can flare a case mouth, but you can never flair a case mouth. However, a particularly talented reloader can demonstrate a flair for flaring case mouths.
