Some thoughts:
Blizzard's standard "non-commercial" ToS probably does genuinely prohibit doing this for a living.
I have read addon code enough to say with some confidence that there exist people whom I'd consider good candidates for a programming job based on reading their addon code -- and others that, well, I wouldn't.
There may be people who are professional programmers, whose work is not publically accessible except for their addons, even if they've been programming for pay for quite a while.
BTW:
http://www.blizzard.com/us/jobopp/designer-ui-wow.html
I wonder if maybe some of the UI developers should be looking at this as an opportunity...