Thats easy to game/exploit with no risk or punishment.
Log on healer alt with bars barely setup. Perform horribly and cause the game to end at lightning speed. Then log on main and enjoy fast ques.
Forcing people to play it on main, teaches them 1. To play it overtime and eventually get some competency at it, additionally to having main mmr/cr linked to it and penalty associated with /afking.
Autofill under my way would require you to fulfill all 6 rounds of lobby. The mmr/cr loss leniency would also not be world ending and easily correctable within a few lobbies anyway.
Yes you can just play a spec you’ve never played. Read the tooltip, press your buttons and use brain.exe its WoW not rocket science.