Specifically regarding the vehicle crew thompsons? Probably a "wow cool" factor/ design choice that has (and I just can't see ever having) any real utility in competitive settings.
Those guys are paper thin, and arguably they need to be for sake of balance. As such, they will never be close combat monsters (or any combat for that matter) and so any further munitions expenditure on them is good money down the drain, especially for close combat weapons. And especially at that laughable price.
Most of the times that's true. I had success with it when I had thompson crew inside and rushed with the stuart into some squads and a wild volks squad appeared from behind the hedges. Received some nasty shots so I jumped out, and wrecked the volks squad. And jumped back in to slowly get out of the danger zone. It can save a vehicle sometimes (especially because the ai won't shoot on an abandoned vehicle unless told again to do so). Still, I agree it has rare use.