The problem I guess comes from if they open it up to all roulettes, it's gonna bottom out the market for materia and make it so it isn't worth the reward anymore... does suck though. That being said, offering the reward to other roulettes would be good. Perhaps they could add like an achievement for doing say... 5'000 of each duty roulette as an adventurer in need. Perhaps something like "The Helping Hand"?
As for joining in progress, I think it might be a good idea. Since the idea of it is to fill up duties in progress that have lost members, it should award EXTRA bonuses (that are available at all times), that you otherwise wouldn't get for joining from the beginning. Also, if you select in-progress, it should PRIORITISE DUTIES IN PROGRESS! Like, seriously, fairly recently I went to join an in-progress duty as tank that had lost a tank (queuing for that individual dungeon), had in-progress selected, it creates a new group for me, while that group still hasn't got a tank. Instead of focusing on the duty without a tank, it creates me a new instance.