If you don't spend time playing a class you forget it. I leveled up my monk to 50 quickly only doing the basic stuff I had to do and I forgot how to play it. Sure next time I take it out for a ride I will need to learn it again or read a guide etc.
I get the people feeling like too many won't know how to play their class but it's a moot point considering people are already pretty bad anyway and it's not caused by just not knowing their class, people who can't dodge or understand game mechanics for example. Has a tank or a healer you really have to learn those and it would probably be a good way to teach them how to play. (Meaning you can't just auto attack in dungeon as a healer or tank you have to do the job and can't really sleep on it)
I like your idea Gendiin, I'm leveling my last class to 50 and Dps queue are a huge pain in the kupo. As someone who like to just complete everything it's a must for me to have them to 50. But I don't expect ever playing my other DPS class much after.