I’m only talking about 5 mans. When there’s only 3 DPS, if you’re lagging behind too much, you’re only in the way.
But even in raid. If you have a spec that’s trailing the 2nd to lowest DPS by a 20% gap in damage by you two, how is that fair to everyone else in the group who had to make up for you doing so much less?
Don’t forget, this is a group a game, you still have a responsibility, and expectancy, to do your fair share and contribute to group content. The other DPS isn’t obligated to have to work harder so that you can spec for arcane missiles.
It only becomes a problem when your fun is negatively affecting everyone elses. In group content, that matters. Spec matters.