I wanted to quickly jump on this thread, I have returned recently and really enjoying the game. I was introduced to Mythic+ through my guild and found it to be heaps of fun, a lot more than raiding. I enjoy the tension and competitive nature of the times and key levels. While I'm no-where near amazing at my class nor am I super geared in comparison to long term players (I've just got over 950), I found that I get invited to groups all the time due to my IO score.
I'm not in the core raiding group of my guild so I have more downtime and I often just do Mythics+, which over the last month has seen me move from +2 to +5 keys all the way up to 2chesting a 16 EOA. Learning all the mechanics quickly on low keys (because my gear was crap) helped me raise my IO. I heal as my main so people standing in things on the ground is really annoying.
Basically, I see people with a 1.5k IO with average gear are much better than running with some high level with no score. Often I see people just standing in crap or not knowing the encouters and it's frustrating. IO, while not perfect in telling the full story, is still a great way to filter out someone who knows the encouter vs someone who will manage to kill themseves or the group by doing something silly.
I have a 2.1k IO score which I'm proud of considering the limits of my skill and playing a non-meta healer. I would love to play higher but right now I know I need more practice to build up to them (plus some heroic raid gear would help too
)