I think we do need workers. I dont even think that Orks having workers is that bizarre. And although stormy is right that a daemon prince on a tractor would be far-fetched, chaos space marines would still have techpriests and such, wouldnt they? even if they didnt, they would probably have slaves working for 'em.
tau have their earth caste looking after industry and agriculture and the eldar im sure have their own workers