Yes I think the humidity for the first 3-4 hours really helps with smoke penetration. After that time the meat has already begun to skin over and the smoke penetration is going to slow down.
I believe that meat is juicy because of the moisture in the meat and that it doesn't get juicier from humidity or adding more liquid to the outside. It just seems to me that humidity, spritzing or mopping just puts moisture on the outside of the meat and then evaporates off. I don't see how it can penetrate deep into a large piece of meat that has formed a skin on the outside.
I have used water and spritzed and I have found I have gotten much juicer meat when I used a dry smoke chamber. I really believe when a dry skin forms on the outside it helps lock in the natural juices of the meat. I smoked a standing rib roast on T-Day in a dry smoke chamber with no water at all and I had placed a pan under it to catch drippings and after 5 hours there were only 4 or 5 drops in the pan but when I sliced it juice ran all over my cutting board.
This is all my opinion from things I have read and the little experimenting I have done with the dry smoke chamber so please don't flame me I'm just throwing out my observations, I'm not saying this is the best way or that everyone will get the same results, just saying I like doing it this way. I'm sure as time goes on I will read of other methods and I will try those too, I am always wanting to try something new!