How important is humidity when smoking a brisket?
According to Big Green Eggic, the main effect of the water pan is help keep the heat even and low in the cooker by:


Providing a buffer mass
Not heating above the boiling point (212 F / 100 C)


This makes it easier to hold even, low temperatures (like 225 F) for long, low and slow cooking.

They claim the humidity change is not really terribly important, as water evaporates from the surface of the foods relatively quickly in any case.

The article Physicist Cracks BBQ Mystery in the Huffington Post blog (which is very interesting in its own right) mentions that many successful competition barbequers are choosing to barbecue at slightly higher temperatures, which would make the entire point of the water pan moot.