Several things and I'll try to give you a quick breakdown.
Water is used in many commercial smokers as a "crutch" to help any user no matter what experience keep a stable temperature. Will they advertise this fact? NO
Water will for the most part make you burn more fuel
If someone can't control temps just with air on a UDS/WSM type of smoker than.....
Also with regards to a UDS type of cooker I want some drippings to flash on my diffuser and some to drop into my fire for added flavor....with a water pan in the way you don't get that effect.