However most of the users of these applications are relatively technically sophisticated and the average desktop user is using standard commercial proprietary software...
This sorta sounds like it is saying that smart computer people use OSS/FS, and dumb non-computer people use proprietary software. Ceteris parabis, anyone who has used a computer for a reasonable amount of time and is capable of absorbing knowledge, should "graduate" from (for example) Windows to Linux.
Also, Skyman, you should have noticed that there were many occasions in the article in which the authors made claims like "open source developers prefer power/functionality to usability". Like WMD's analysis of your comparison of iTunes and xmms, we see two completely different ideas of what constitutes a good program.
And if you think about it for a second, the definition of usability is only a real issue for amatuers. Once you have been banging on a program for awhile, usability is no longer your concern. At that point, power becomes more important. My opinion is that using "usable" programs can cripple your ability to learn powerful software with an "unusable" interface; similarly, if you take a while longer to figure out the powerful stuff, you can master the easy stuff quickly.
Finally, I would like to head off any potential arguments by saying that the authors did not truly address the question of whether it is even possible to have power and usability in the same program. On can argue that Photoshop is more usable and more powerful than gimp. But the gimp has only been in development for a year or two, while Photoshop has been around for much longer - the proprietary foundations of Photoshop were patented by John Warnock back in the 70s (based on stuff he developed at Xerox-PARC). In reality, one cannot even compare gimp and Photoshop for neither power nor usability. So the whole argument is moot anyway.
