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Controversial question and you are going to hear answers both ways. Personally I generally use a water pan with just water in it. I feel the water pan provides no flavor with additives so I put the additives into the spritz which does actually touch the smoked product. Just my humble opinion.....
Water pans are there for two reasons: First, it helps regulate heat in the smoker by virtue of the evaporation process removing heat from the airflow.
Second, while it does not "moisten" the meat, the water vapor/steam it produces keeps some of the moisture from evaporating from the meat by keeping a close to saturated atmosphere in the smoker.
If you have trouble maintaining high enough temps, don't use a water pan, but DO mop more often after the rub has set on the meat.