At this point, you should have far more gear than any dps requirement actually needs and if you really are lacking dps, your raid has a l2p issue. That said, there are certainly multiple reasons that some one not in a top tier progression guild might want another class / some one to reroll another class instead of a rogue, and I would think that it is even far more likely if its a 10m guild.
Fine, but uninteresting.
Don't let some of these people (such as the above) scare you. I play a recently "dinged" 90 Rogue (Assassination). My gear is a mix of greens and blue's. The highest ilvl on any item I have is a couple 463's, or heroic 5-man level stuff. Currently I have no problem with damage when grouped with people in similar gear. On large groups or trash I can hold my own. And on bosses I'm usually 5%-10% higher than the next person in damage. The only place I suffer is with small groups of trash, say 3-4 npc's. This is because Assassination has no viable "cleave" and the group is really too small to make FoK worth anything.
Sorry to tell you this, but if that is true then the people you are playing with suck. Our dps in blue gear was well documented in logs during the first week of raids this expansion. But yeah if he also is playing with people far below his skill level, he may be able to be 5-10% ahead. My statements (and those who said similar things) were based on the assumption of roughly equal skill and gear.