I'm in the unfortunate situation of being de-scoped at the moment (that is, part of my job just got cut, so now I get the joy of going out and searching for more work), so I went up on our company web site looking for openings and I found a couple positions that were looking for usability.
However, what they want is one person who can both develop software and do usability. That's not what I am, and for good reason. While I love computers and what they can do for me, I am at heart a people person. I thrive on trying to understand people better so that I can make tools that will better serve the person using them. I have a strong sense of empathy, which I think is a requirement for a good usability engineer.
This is not typically true of software developers. I've got loads of them as friends and I adore them. They are intelligent and usually have fantastic senses of humor, but they would prefer a computer over in-person interaction anyday. I have noticed they occasionally actually even take pleasure in making things difficult for the user (usually as some form of joke).
So what am I saying here? I'm saying (and this is a generalization, but ...) that developers are not people people. They thrive on making electronic devices do new things, but they don't particularly rejoice in studying human behavior. Human behavior is what drives usability of any product or interface. Human cognitive abilities, reactions, preferences, limitations--these are the determining factors that effective user interfaces must take into account. (Most) software developers don't excel at those factors, but usability professionals do. (Of course, there are exceptions--I know a few developers who are also excellent at user interfaces.)
The other problem with having software developers make user interfaces is they develop for people like themselves. What percentage of our population is like a developer? Approximately 20%. Software developers are the type of people who want to know how it works. When they see a cool web application, they try to figure out how it was built. The other 80% of us just want it to work and couldn't care less how it works.
Perhaps we need to get out of the old mold of believing that our software developers should develop interfaces and try a new approach. One that includes someone who lives and breathes user experience, but maybe doesn't know (or care) so much about the inner workings of the code behind the interface. This approach also saves precious developer time (which is usually very expensive). Let's stop wasting our developers' time by asking them to do something that is against their nature.