So everyone voting in here for increased realism is certain that squads weren't outfitted with similar weapons in real life? (minus the mkb and other rare weapons- I agree there) I certainly am not an expert, but I would think that smgs would be a popular choice in urban environments...
To be fair, if you are into realism, its not the "was it possible" as much as "was it common".
In game you want to be a member of a stereotypical Soviet or Axis platoon, not some rare or non-existent experimental platoon. I could however imagine that when put into battle for testing purposes that there would be a lot more MkB's per platoon than people are thinking.
When an army is testing a new weapon in the field, they are not testing how accurate it is. They are testing how it should be used as part of the platoon's battlefield tactics. Since the weapon is designed to replace both the rifle and the smg, it seems to me that any platoon experimenting with their use would actually have nothing but MkB's and MG's.
If not, they wouldn't really be testing to see if it can actually replace the rifle and the smg.
So while it may not be strcitly historically accurate for them to be there, or even in terms of capability, the numbers of them isn't that unusual. In the battle, we are one platoon, or less, of soldiers. That's 30 men. Replace all the non-MG weapons with the MkB and you would have 27 of them.
So what we have is less overpowering than what was meant to happen (at least eventually) in the real world. It's unrealistic not to have more of them, in other words, but I'm sure the RO1 fans won't be too bothered by that.