I always use a water pan....especially if cooking at lower temps. The idea is it's a heat sink that helps stabilize temps.
Of course, I'm cooking on a water smoker. WSM's and ECB's are water-type smokers.
The pros on here who use offsets or UDS's will likely tell you that they don't ever use water.
Think it just depends on what you're cookin on.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team