Sometimes even Apple doesn't get that. There was a great example of this principle in one of the slides the Engadget blogger took during Jobs's presentation (http://www.engadget.com/2010/01/27/live-from-the-apple-table...), where the 10:57AM slide shows the iBooks application allowing the user to "change the font (to) whatever you want."
Um, thanks but no thanks, Steve. Book publishers hire type designers to come up with appropriate fonts, and they hire other skilled designers to select and use them. Leaving this choice up to the reader is not a feature, but an aesthetic regression at best and a dereliction of duty at worst. It's like bundling fine art with a paint-by-numbers set.
This is exactly how MS Office ends up with 30 different selectable toolbars that nobody knows how to use. It was a splash of cold water to the face to see Apple making the same sort of mistake.