I would like to point out that the game CURRENTLY has a system basically identical to this. You flag yourself LFG and type your role in the comment and the list itself shows ur class and level. Heck you can even kinda say what u want to group for. But nobody uses it.
The problem is that once you look into lfg and see the 8 dps and no healer or tank, people tend to get very discouraged about it. And unless you sit there and refresh it constantly (essentially manually updating the queue) you are likely to miss someone who just logs on, flags himself for a few minutes, then logs off.
This is not a problem with the system but people. Just as they would be in queue waiting for a healer they could invite the people and wait for a healer to join but they don't want to do that do they?