Attire is about perception. If you are well dressed people automatically see that you are conscientious about your appearance and as such, by suggestion, other things - including your profession. This is important if you are trying to create an image about your professionalism to a customer, or a superior, but plugged in the only customers I need to prove my professionalism to are at the other end of a radio. I fail to see what an ATC wears to work has any bearing whatsoever on how well he/she does their job.