I think the water pan helps stabilize the heat in the cooker. I don't have a pitmaker but I have a Stumps. Different concept but I've never had issues with temp control. I set my guru and it stays at the set temp the entire cook.
You might want to talk to some owners of Backwoods cookers. Those cookers also use a water pan.
When I was in the market for a cooker, I looked at the Backwoods but I didn't want a cooker with a water pan. So that's one reason I chose the Stumps.
Bubba Keg / Stumps Baby / Super Fast Yellow Thermapen / Lodge Cast Iron Grill / Coon Ass Pizza Oven