Not entirely true. It is not illegal to use F/A lower components in a semi lower. Plenty of people running F/A hammers for example, because they think the extra mass increases reliability. Perfectly legal. As for the adding of weight to the system, you must remember, that the system was originally designed with the heavier F/A carrier. So based purely on that fact, I'd argue that everyone should be using one.
The other main benefits to using a F/A carrier:
1) Shrouded Firing Pin. Some semi-auto carriers still have the lower portion of material below the collar of the firing pin removed. This is a throw back from the 70's when Colt altered some of their parts (hammer, firing pin and carrier) to induce a malfunction if the disconnector was removed. The presence of the shroud also takes the stress off of the firing pin during cocking.
2) Dwell Time. This concept goes hand in hand with increasing buffer weight and really only becomes a factor in shorter carbines. Dwell time is the amount of time the system maintains pressure while the bullet is PAST the gas port but still in the barrel. The dwell is longer on a 20" or 16" than a 10.5" barrel. By adding mass to the carrier (and buffer), the system is slowed, increasing the amount of time the bolt stays locked. What this can translate into, is greater reliability with weaker or worn extractors/springs, among other things.
3) Bolt Bounce. This is another interesting concept that also pertains to buffer weight and carrier mass. With lighter components, during chambering, the bolt carrier will actually bounce back when it makes contact with the barrel extension. This is generally not a great cause for concern in semi-auto systems but in an automatic firearm, can result in misfires and light primer strikes.
Lastly, I should clarify that simply adding weight to your rifle is not the solution for every length and gas system. In some cases, too heavy a buffer for example, will cause last round hold open issues.
