I think the problem would be that it would stop people from needing to run the dungeons and make them even more deserted.

Say there's one person who needs it to proceed, and three more people join just for the drops. Party is complete and they can head in.

Say the same person is looking for dungeon completion to proceed mission-wise in your case, and the three people who would otherwise have joined and given the first guy progression would now just stand outside and buy the gear they need and leave again, essentially leaving the first person high and dry.

Brayflox (and many others) are terrible dungeons. No one runs those for fun, they run them for the drops. If you take away the need to run the dungeons for the loot, no one is going to do the dungeons anymore, even if there was a prerequisite number of runs they would have had to to before that. People take the path of least resistance.