Not sure how everyone else does their guild recruiting. In my guild we say that we are looking for a healer. If a Priest applies to the guild - we let him know we are looking for healers. He says, "OK", and joins up. Later on, if he decides he wants to do DPS, he gets moved to the back of the queue with the other DPSers.
Same goes for all raid roles. Just because I happen to need 1 more tank, or 1 fewer healer for a stupid trash pull, does not warrant a player to roll on a piece of gear that drops from the boss.
Just because you can change specs on the fly does not mean that players will not have 1 "primary" spec. Sure there will be players out there with the hours and resources available to gear up, and get good at both specs. But I imagine most players will not.
Failure is not an option. It comes bundled with the software.