The only reason I see it as a possible advantage is the fact its given as a doctrine perk, it must bring some use right? Or again, do I put too much faith in relic and it is simply supposed to be there to protect your vehicles with greater DPS when you decrew.
Too much faith
They are pretty worthless. I've heard people suggest using them as a ghetto drive-by where you circle behind an AT gun or whatever and hop out to kill the gun crew faster with the Tommies (especially if we're talking about a TD crew). But this is an unnecessary risk. Simply put they aren't worth it because:
1. The Muni is better spent on pretty much anything else (LMGs are king in COH2) not to mention mines, smoke, call-in Arty, P47 Strike, you name it.
2. Even vetted, vehicle crews are insanely squishy.
3. Vehicle Vet is tied to the crew's EXP (so your vehicle would lose all Vet unless you swap crew right away).
4. Mentioned above, REs don't auto-repair. Adds micro at a point in the game where USF player is likely already attention strapped.
If you really want SMGs best to just get Paratroopers or Assault Engis. At least then you get cooked grenades and/or Demo Charges (again, better use of Munis)