Strange proposal. I'm a solo player and I like to play, say, as a MCH. I find myself in a group where I see another 3 MCH. Either they or I will have to change jobs. And most likely it will be me. And how can I play my favorite job in this situation, if every time I jump into random group and system will force me to switch to a class that I don’t know or don’t like? Expecting people to switch for increased damage or a buff is very ridiculous. Expecting premades to do this is doubly absurd. The entire burden of changing jobs will fall on the shoulders of solo players.
