It is easy for managers and customers to appreciate what they can see. I have seen many GUI developers who are average programmers with minimal knowledge of design principles or other programming idioms. However, these shortcomings often go unnoticed, specially by management and customers, if the programmer can create an impressive looking user interface. So much so that many GUI developers I know spend hours beautifying the GUI at the expense of writing bad, unmaintainable code. On the other hand, middle tier programmers who develop APIs or business functionality or database code (SQLs etc.) are at a disadvantage as there is nothing tangible to showcase. Perhaps a code reviewer or an architect may appreciate the elegance, good design, scalability etc. of code but it means nothing to the outside world. Your code may run for years without breaking, may be very easy to maintain and have good performance, yet it never elicits the 'wow' that a slick looking GUI does.In my opinion, a corollary to this is (and I am going to get heavily downvoted for this, I know) that there is less motivation for a GUI programmer to write good clean code.EDIT: I must explain here that by GUI programmer, I don't mean a full-fledged web/GUI designer but a front-end programmer e.g., a java-swing programmer.Does the rest of the community agree ? 
Do GUI programmers have an undue advantage over others?
gui
I think I see your point, but I suspect that there is also an opposite issue to consider.Essentially, I believe you are suggesting that, because the UI is the element of the application 'in the face' of the end users, the UI developers enjoy a higher visibility than the team members working in deeper layers of the app.Certainly I agree that there may be a higher visibility. For instance, developers working on the UI elements may get to interact with the end users more often (arguably, for good reasons, since they do focus on the Human/Computer Interaction aspect).Yet, I think that the higher visibility comes in play even in cases when there is a problem. For instance, end users are very likely to report issues as 'GUI Issues' even when they are not.It may all boil down to perception, and a mature organization should be able to recognize values, virtues and weaknesses of the various team members independently from which layer of the app they work on.  A mature organization may also have moved beyond distinctions like 'UI developer' and 'business layer developer', recognizing they are all team members anyway, with different expertise perhaps, but always trying to educate each other on those areas of expertise.