Yeah, it kinda does.
Just because you can still play the spec and still kill the boss doesn't mean the spec isn't bad. The spec is bad.
When I'm parsing 96% and the Shadow Priest parsing at 55% is doing 100k more DPS than I am, the spec is bad.
If I was playing a different class we would be more likely to kill the boss. That is true no matter where your guild ranks.
Every DPS check is a challenge for every guild until they get it, and playing a class that has poor DPS makes DPS checks harder whether you are top 100 or top 10,000.
Of course it is true to say that if other people were pulling their weight then we'd be more likely to kill the boss, but that's no more true than saying if the spec was pulling its weight we would also be more likely to kill the boss.
So please stop patronisingly trying to tell people it doesn't matter. It does matter.