I've dealt with this mechanic in DCUO.
Someone queues as a tank or healer to get a quick instance pop and then switches to dps.
More often than not one of the dps then agree to switch to heals rather than wait even longer for another healer.
The abuser gets a fast queue time and makes someone who has waited patiently to play as DPS switch to something they didn't want to play.

If someone was to do this three or four times before not getting kicked they would still get a group much quicker as a dps.

Everyone should realize in 2014 that if a MMO mechanic can be abused/exploited it will.