Here's an interesting article on pros and cons of various systems of raising chickens, including tractors and free ranging.
I don't necessarily agree with all his conclusions, but he does a good job at looking at the factors involved.
Interesting article! Made me re-think my chicken tractor design, which is still in the planning stage. I think, I'll get something like this tractor http://www.backyardchickens.com/a/egghead-jrs-chicken-coop-tractor plus some temporary fence with bird netting across. The whole arrangement could be moved around on my property (2/3 acre) ever so often. Does this sound OK? I'm planning to get 3-4 chickens for eggs only; RIR, maybe?
Personally, I think so. They will kill every inch of grass in two or three days, most likely. But I think it is much better than my stationary coop. I really want to have the option of easily moving my coop around...