I'm aware your original post specified an MP5K- and I still stand by my point. It's extra weight that would not be necessary for a pistol only loadout, and to top it the weapon is made of poor plastic that may not handle knocks and bangs of CQB. I've seen people accidentally drop weapons down stairwells in CQB, and the Well MP5K probably wouldn't handle that sort of treatment.
As well as this, if you put the weapon on a 1 point sling and kept it by your side as you moved with pistols in both hands, it would flop around a fair bit- I'm sure there are holsters out there for it, but then it's even more unnecessary expenditure on a backup which will just slow you down in CQB.
If you do come against something heavy in a skirmish, you probably wouldn't be aware of it's existence in CQB until too late- and in woodland you could leave a support gunner or hi cap spraying skirmisher to your better-armed team mates. And even then, you could still potentially 'kill' the support gunner through quick movement or moving to a position where they cannot engage you effectively.
I still think it's pointless taking a 3rd weapon into battle on your person while playing in a role which requires lightness and quick movement. Sure, the Well won't weigh a lot, but combine it with the weight of your rig, the 2 pistols, all their mags, speed loaders, gas, holsters etc- it adds on unnecessary weight.