Deeje says (in discussing various UI treatments in Apple's current lineup) "Fonts serve no purpose other than to distinguish one set of characters from another."

Err, sorry bud, I'm not following you here. Of course font faces differ from one another in appearance (lest they have no reason to exist). But to say that Helvetica exists only to be different than Courier New misses the real reason(s) for font face variation; to improve readability in different sizes and mediums, to convey personality or emotion, to different uses (e.g., headline [traditionally sans-serif] vs. copy [traditionally serif]), etc.

UI variation should serve a purpose, even if that goal is simply to "be different".