The minor boost I had in mind was like from +10% to 25%. (Special Ops perk gives +100%.) Yes, marines are already equipped (only they can board ships in the first place), but nothing says you cannot equip them with superior equipment above and beyond the baseline. Yes, they may become "mandatory" if you board much, and if you have plenty of cargo space. When I board ships, cargo space is at a premium, I bring three or so Atlas to take all of the loot from multiple battles in hostile systems, and I manage to fill up space after a few encounters. If weapons boosted marine effectiveness, the trick is to give enough so that it is worth the loss of cargo space, but not too much as to make it mandatory.
Also, hand weapons have the minor problem of being illegal in some factions' space. If you get cargo scans, you risk losing them.
That said, I do not mind weapons having an additional use aside from boosting marines or mundane trade.