Most airlines say no backpacks...... er, most Majors anyway. So, if you want to be a Major airline pilot one day, start acting one.
Professionalism. You either have it, or you should do something else (go back to school). Wearing backpacks to work only encourages your management to treat and pay you like a monkey. Unfortunately, it makes the rest of us look bad too. So, if you are wearing a backpack - expect me to tell you that you look like a moron in the airport (because you do).
Is it ok to have the bag attached to a rollerboard an not wear it? Is there in fact a correlation between professionalism and the shape of a bag?