I think part of the disagreement I have with Vaaish's interpretation of the Rapier is that I don't believe it should be a Firefight support weapon at all. To me, the Rapier Laser Destroyer is a science-fiction version of light antitank weapons such as the
37mm M3. It's designed as an antitank weapon, but can be fired at infantry as well. What it is not, however, is useful in a close-in assault involving small arms fire and grenades.
The M3 is effective out to a thousand yards, and is most effective when used as an ambush weapon against enemy armor. It is not designed to be part of an offensive rush, as it cannot be fired effectively while remaining mobile. FF4+ is the same value as a squad of Tactical marines, or highly trained and equipped Stormtroopers. While the Rapier should be significantly better than those troops at destroying armor from a fixed position (AT4+) it should not be their equal in a close, no-holds barred engagement with enemy forces. That's not what it should be used for at all, and making it into one more Firefight-based unit eliminates the flavor of an infantry-mobile antitank weapon.
EDIT: Likewise, that's also why I feel it would make more sense either as a standalone formation. It's not a weapon that should ever be committed to an engagement, but used at distance to prepare a target for attack by another formation.