My 2 cents.
After reading all the post I would say I'm surprised but this is how human behavior works, most people no matter how smart or intelligent (and there are some intelligently written post) seems to lack understanding.
First, I agree with the OP in that N7 does matter, along with the score. What so many seem to fail to grasp is there's always going to be exceptions. That low level N7 but is really good, or that high level N7 that is really bad. I simply liken it like the PSP or SP where you buy it for a chance at that UR, its random but with a percentage involved. A good low level N7 is like getting that UR from the PSP but most of the time it seems to be simple CC (or ammo packs,) you know that not so good player. I've play mostly with randoms and N7 less than about 200s are rather, for the most part with a few exceptions, rather annoying. And as the N7 rises, the likelyhood or chance of better players, better teamwork, better understanding of the game mechanics also seems to rise, again with some exceptions.
I would say this, that there is a strong likelyhood of there being more bad players at low N7 than their are at higher N7. It doesn't matter if there is a few good players sprinkled among the low N7 or if there is a few bad players among the higher N7. A high N7 most likely means a high amount of play time and experience (please stop giving exceptions they don't matter). The larger playerbase of low N7 can mean noob, inexperience player, someone not to familiar with MP or a player that don't want to promote, but regardless because low N7 have the noob, lack of experience playersbase in their numbers they are going to be more likely, on average, to be less optimal at the game than the higher N7 which has more skilled experience players in a much smaller pool.
Again it is kinda random, but N7 does matter to a point. You are essentially gambling and picking the best odds. And if picking the best odds matter, than N7 matters. It doesn't matter if the best player is N7 240 and the worst player is N7 24,000. N7 is not a direct correlation or indicator of skill of each and every player, it matters in that its a number that helps us guestimate the match's outcome and if we are willing to take said chance.
Also I top the score with support classes, while giving support. A justicar with defense bubble. A SE with a decoy. A GE with a fully specced heal turret. No a support class probably won't get in the top against a heavy damage dealing class that is understandable but usually the best player that game is the one that topped the score board. Again, maybe a few exceptions.
TL;DR
N7 is not a direct correlation or indicator of skill of each and every player, it matters in that its a number that helps us guestimate the match's outcome and if we are willing to take said chance.