I personally don't use water pan, but I'm cold smoking - bringing more moisture in is counterproductive as it defeats purpose of preserving meat by reducing water content.
If smoking with temps above 100F using water pan makes perfect sense. According to Marianski bros., the ideal smoker humidity for warm/hot smoking is 80%. In dryer climates and considering that humidity is in inverse relationship with temperature adding extra moisture as temps are getting higher in the smoker is probably a very good idea. One thing to watch for would be not to got overboard with water and let surface of the meat get wet while smoking it.
BTW as nepas mentioned, Marianski has a para in his book about "wet smoking" (adding hot steam) which is used by commercial sausage makers, but as he succinctly puts it, not for reason of making better quality product but "less water = less weigth = less profit". He should know as he worked in that industry for a while.
I also adhere to his strict rule to never, ever let smoker temp get above 170F for more than few minutes. But that's not the topic for this thread and could spark some controversy. 
Cheers and happy sausage smoking.