People usually organize m+ groups so they can pretty much "trade" the carrys for alts. For an example, I log to my main when my friend log to his shitty just dinged alt and vice versa.
One problem with taking people from your raid core to m+ on their mains is that they may perform insanely bad. If our group has ran 200+ times some specific dungeon with same same players everyone knowing how tank pulls etc who stops and what without communicating those things, why should we make it harder by taking some dipshit who has done only m+ 15 once per week and is first time in +20 run? Of course we could carry them on our mains, but another problem is that we usually rotate mains and good geared alts on groups so we have 1-2 carry dps and 1-2 shit dps depending on the key level.
Then the m+ pushes, even if I were ready to carry some core raider nub thru high m+, that doesn't mean that tank, healer and another dps would be willing to do that. I personally love m+, but I play only with players who I know I can trust on. I don't care about few personal fails, but some random WW monk dying two times per trash pull is pretty annoying (damn melees, they have no brains).