As someone who is not a huge M+ player, im struggling to find anything against your final point. There have been many times where i have had, say, a +12 key on an alt and some IRL mates want to do some keys but only have some shitty +2/3s. I think to myself "welp, bobs pissed as a chook, marry is on her third glass of wine, and i think karls falling asleep - not really in the mood to deplete my +12 right now, I wish we could just do a +7". Obviously a hypothetical example, but there have been plenty of similar situations when we mix our main roster with alts, or irl friends etc.
There very well could be a negative consequence im not thinking of though, so feel free to point it out if anyone can think of something"