The roulettes can absolutely be improved. There is nothing about having more control over the outcome of the roulette that goes against its purpose (and having choice is just an extension of having multiple roulettes - if choice didn't work then mentor roulette would be the only one), and yes that includes the purpose of helping fill queues. If better roulette outcomes increase participation, and people with different preferences queue you could very easily end up with a net increase in roulette population while also preventing repetition, monotony, and general dissatisfaction.

A hypothetical roulette bias system or duty blacklist would need to be evaluated carefully of course. There is a chance that one particular duty that is universally disliked it avoided, which would make queueing for it difficult, but even in that case that's not completely a bad thing. That's a big flag that something might be wrong with that duty.