They may very well be turning down a player who may not need wow gold or even want the drop in question.
Even if they do, people are often willing to pass to someone who's been trying to get a drop for weeks. The dungeons aren't going anywhere. I think we can all be fairly certain that Blizzard intends to keep them in the game. And if I'm irritated, a somewhat gentler version of: Haven't we been waiting long enough to get this run going?
Personally I don't think it's really all that fair to discriminate against potential group members on the basis of loot they do or don't need, especially when you're just tossing a 5-man/heroic run together, but I will also admit that I can afford to be fairly cavalier about it. Tanks generally don't have issues getting a run, so I may very well be underestimating the frustration factor of a DPS who has to spend significantly more time than I do getting a group and a shot at a drop they may already have lost multiple times. If I were in that position, I wouldn't necessarily feel great about losing a drop for the umpteenth time just because my group leader couldn't be arsed to ask about it before adding people.

Requests like these are cropping up a lot more lately, particularly with players competing against the plethora of Death Knights now leveling through Outland and Northrend. This has run the gamut from a DPS Warrior who didn't want to risk losing a ring to a DPS Death Knight, to a tanking Paladin who was heartily sick of doing the same dungeon a million times waiting for boots to drop, to a Death Knight who was desperate for wow gold anything better than leveling greens but kept losing rolls to people whose toons spent months at 70. I understand the impulse, but sometimes people will get fairly nasty over having to accommodate potential competition, and I have actually dropped a heroic group where this occurred. When a DPS DK starts demanding that I uninvite a fellow DPS DK from a group because the player is certain to roll on "his" weapon, I take that as a sign for me to get the hell out of Dodge. Congrats, buddy; now you need another DPS and a tank.

I don't think it's right to put a group leader in the position of having to judge who's more "deserving" of an item that may not drop anyway, but I can't pretend that that's not going to happen, or that the leader doesn't have some measure of responsibility. In an ideal world, the group wouldn't dump the job on the wow gold leader, and would be able to suggest available players for all the needed slots in a group with a minimum of loot competition (either because people didn't need certain drops or were willing to pass) -- but I also won't pretend that this happens all the time either.