The link between skill at a class and raid difficulty is a false dichotomy. Even in one single guild such as Method, you see some players who constantly outperform someone else in the guild who plays the same spec. You get varying skill levels at every level of raiding, and I'd argue the difference in skill between a member of Method and a member of a guild who killed Kargath Mythic today is much greater than the difference in skill between the player in the guild who killed Kargath Mythic today and a player in a guild who is 3/7 heroic.
I'm not comparing a good rogue to a bad player. No matter what level you're playing at, you will always see wildly different levels of skill. The point is that the majority of players are not raiding mythic, so if you ask someone who is 3/7 heroic whether he cares about whether he beats his friend in the same guild then you'll either get a yes or a no depending on the type of guild and the type of player, but the key is that you can get a yes. Being competitive does not mean you have to be #1 in world of logs, it can mean being your rival on your realm group or reaching a world rank of 15,000 this tier. It might sound silly if you're a mythic raider but that style of competition exists for a lot of people. Less skilled players do like to be competitive too, and it can really matter to them.
I'm absolutely dire at Halo but I still like to try and win, y'know?