Those that care to improve can find relevant numbers on the internet or ask others for useful information. Those that don't care will ignore it either way.
And if you can't guarantee that the other person is also playing optimally (which you can't or else this wouldn't be a big argument), then that context is worthless.The only way to gain the context is constantly going into DF with other dps, check their gear and see how they perform in relation to you.
I mean, honestly, the whole argument that you need context in DF groups to improve is defeated by the same reasoning that a majority of players are low quality and do bad DPS. Either context in duty finder groups is important because you're being grouped consistently with people who ARE pulling good numbers, or the context in duty finder groups is unimportant because a majority of what you'll get are other bad players. Pick one. You can't have both.
Which not everybody knows. Especially if they don't already use the internet to know the correct numbers.(honestly it isn't THAT much of a difference)
They have as much a right to be in that group as you do if you used DF. You don't own the group. And if you ever tried to kick a DPS for pulling low numbers in a run I'm tanking, you'd shortly find yourself outside the dungeon.they can gtfo of my group for wasting my time.