Water in the pan creates a more humid environment. Some people with *way* too much time on their hands did some studies and showed that the extra humidity can cut down on the stall. You can get perfectly moist meat without the water, and you can decrease the length of the stall by cooking at higher temps.
I cook chicken, turkey, steaks, hamburgers, hotdogs, and anything else that's "high heat" (above 300*) on the BGE with no water pan. The BGE seems to hold a reasonably moist environment on its own (though probably not near as moist as a smoker with a water pan).
I cook ribs, butts, briskets, and anything else "low and slow" in either the BWS or WSM, and typically at about 235*. I've never cooked in either of those without water in the pan.
To each his own. One day, maybe I'll try some no-water cooking, but I'm VERY happy with my results, and my smokers hum along on temp beautifully.