Why not allow class changing during duty finder dungeons?

[Wilan]

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.

Yeah, and wouldn't accept you in their party if you didn't have all the required jobs exped and geared.
Words cannot express how glad I was that dungeon job switching was gone in 2.0, and I see no reason to make it come back.