Think about this....

0 tanks in queue, 2 healers, 10 dps in a queue for a 4 man dungeon

Current in need: tank

According to your suggestion: after 2 minutes, in need: dps

Everyone sees that and queues as dps

0 tanks in queue, 3 healers, 30 dps in queue

dps now have to wait ages

you now get some 'bonus' exp but have to wait twice as long since everyone wants the 'bonus'.