I've cooked a lot of chicken w/o a water pan and didn't have any issues. You will get a bit of smoke from the drippings vaporizing when they hit the coals, but I haven't noticed it causing an off taste.
On a side note, I think the whole beer can chicken thing is just a gimmick. The fluid in the can doesn't get hot enough to evaporate and flavor the chicken, at least it didn't the few times I tried it a long time ago. What I like to do is insert the wire stand that the beer can sets in into the chicken empty without a can of anything, it holds the cavity open so the heat and smoke can get inside the bird.
...On a side note, I think the whole beer can chicken thing is just a gimmick...
What do you guys think about beer can chicken on the WSM with no pan and just have drippings fall directly on the charcoal? Will this cook it faster or just char the legs and make a big mess... Thank you in advance.