I had the same question a year or so ago. I have a real nice Weber gas grill I use for daily stuff, and that played into my decision to get the WSM. I just didn't see myself firing up the BGE to do anything other than smoke. And the WSM does that for less money and does a damn fine job.
18.5" WSM ; Weber E-330 ; Weber Q200 (tailgating) . . . Guess I'm a Weber fanboy!