Why not allow class changing during duty finder dungeons?

[Icarian]

I'm all for SE letting you change jobs if the party is not full (someone leaves/DCs or whatever), but letting you change it whenever you want to will lead exactly to what's below

I guess it's because it might lead to situations similar to 1.0 where groups changed to optimal setups for every boss inside one dungeon.

They would have to re-tune dungeons because of that, like Wilan says, that's exactly what happened in 1.0.