we use a labor staffing service regularly when we pick up large installs and need temporary labor. It works for us, we pay one flat rate and everything is covered, pay, taxes, wc etc.. If a guy doesn't work well we simply call the service and tell them to send someone different the next day. We usually find a few guys that work real well and as long as we give a few days notice then we can request them for future work.
We have also hired one of our laborers from the labor service. It's a good way to see if an employee will work out before you hire them. You just have to watch out because some staffing service will charge you if you end up direct hiring one of their guys.
One thing I have learnt with temp labors is to offer them a bonus. They make crap for pay, so I let them know that if they bust their *** all day/week I will throw them some extra cash that the service doesn't need to know about. That always encourages them to work even harder.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!