In order to make them worth it they must be a sizeable upgrade for rifles. And when they are that means that RET with these upgrades will be able to out DPS and win for cost against squads they shouldn't be beating.
The DPS for picked-up weapons varies with the amount of Manpower the unit that picks it up. For example, Riflemen that cost 280 PM with BARs will do MUCH more damage than Rear Echelons with a BAR - REs cost 140, just half that, and thus will do less damage. Withs BARs in the poor state that they're in at this moment, the "upgrade" will be nothing more than 60 or 120 wasted munitions.
(which, by the way, is why Bazookas on REs are somewhat effective - they don't lose damage per shot)
And, by the way, don't expect the Rear Echelons with M2 Carbines to become absolute murders on the battlefield - they're still only engineers, and with the M2s, are comparable to Pioneers. Slightly less DPS per models, but has less DPS drop-off at range (pretty much uses the Assault Rifle weapon model). Nothing too OP, it just allows they to become effective at something.
@DakkaIsMagic
Really? The REs for me drop like flies when they try to advance on the enemy. And when they stay put, they just get out-DPS'ed by everything else on the battlefield. And Fighing Position, as stated before, are built like a house of straw and, unless the RE is Vet 2 with 5 men, will get 1-shotted by ANY explosive if it lands inside of the position.