for premades I could understand...
iirc we could already do that in treasure hunt dungeon...

but for pugs would have different story...
I could imagine player will queue as any [role in needed] for the bonus and back to their main inside the duty...
then create a drama over who should be healer or tank or some specific dps over the other...
or worse, asking why (insert a player name) using this job instead of the other job over small niche things or based on some rumors about meta things...
then wonder why it's taking forever to kill because of certain job composition: meta but barely understand how the job works / under geared...
poor sprout will be excluded in a duty because they didn't use/have specific job for this duty...
and they talk in different language you don't understand...

Need/greed loot will have no meanings since everyone could just change job and need...

report of harassment everywhere because someone don't want to change the job the other asked... (or don't want to fill the role they queued for)
banned player everywhere...
complains again in the forums about the toxicity of community, and ask for these switching job in duty to be removed...

yea, this example is a bit over, but there's nothing too extreme for pugs scenario...