I tried it once on a cool fall day (low 50s/upper 40s) and breezy, and I could not get the temp above 150 or so, even with a full load of coals and a DigiQ running constantly. I can't say for sure that the empty water pan was the culprit, but now I fill it 2/3 of the way with playground sand and block the wind. Temp holds steady, even in single-digit temps.
My thought is that, once it is a temp, the sand (or water) is a big 250 degree thing that helps to regulate the temp inside the smoker.
EDIT: I bet I could get away with no pan in, say, July when the outside temps are nice and warm.