M+ score reflects your experience with high keys. You know.. like understanding which abilities start to one shot you at what point, how big pulls are likely to be, what pulls you can just yolo bomb which ones you have to handle a bit more carefully, when you should and shouldn't use cds, etc.
The fact that you and other people seem to think that his is useless and hate on the M+ score just shows how effective it is in keeping dead weights out.
p.s.
M+ score is irrelevant if you have friends/guild.