I admit to being new to using a water smoker. However, for a few years now, I've used a UDS I built after researching the BDS, which is very similiar to the larger wsm, just with NO pan.
The disadvantage of no water pan in a vertical smoker is that the temp. WILL fluctuate more if you are taking the lid off to mess with meat OR the wind is variable. If I want to cook on my wsm hotter than 300 degrees, sure, I won't use water in the pan. However, it just makes sense to me to use it when cooking low and slow. I cooked a couple of butts recently in temps below freezing for the entire cook. I didn't add lump or water until the 12 hour mark, and I was able to sleep all night without worrying about spikes due to shifting wind. It's just a bit of peace of mind when you want it to stay rock steady, and you don't have to worry about the pit temp ramping up incredibly high when you happen to take longer than you want to in removing a rack to check meat underneath, etc.
One reason why the UDS is such a great chicken cooker is that temps naturally rise real fast when you take the top off to turn pieces and start checking. This helps the skin finish cooking, and it can even get crisp if you leave the top off long enough. For low and slow, I just like a more well "behaved" pit.
Now, I would like to know what the advantage of the flower pot base on top of the water pan is.
Dave
UDS, wsm, weber kettle, char-griller