While it does sound like a good idea, there are some problems that do arise.

1) The idea of voting definitely fixes a lot of potential griefing.

2) You should only be able to switch to a class that is missing.
If a tank leaves, someone can only switch to a tank class.

3) This is the biggest issue I can find.
If a DPS class can switch to another, that brings a whole new level of griefing.

Say some group in the DF wants to SR (Brayflox, for example), if you're a DRG queuing, and they want a BLM, they can simply vote kick you for not having the ability to switch to their "required" class.
It would definitely happen frequently, considering the abuse coming from vote kick/abandon already.

I can imagine it already...Oh, the horror.