Michael Tsai (author of SpamSieve) notes that as OSX has iterated, the pinstripes have faded and faded to nothingness, in See the cat? See the stripes?.

The reduction of the stripes and gratuitous transparency are, to me, among the most important changes that Apple has made to the OS. As with some of the other OS X improvements between the public beta and 10.4, celebrating this is kind of like thanking the bully for not beating you up anymore. It doesn’t really make sense, but you’re so happy that the bruises are healing.

Certainly I complained like mad about the unreadability of screens in the beta -“like they’re smeared with margarine”, I said at the time. But I think Michael’s got it just right here:

My theory is that the stripes were a gimmick to encourage carbonization by making Classic applications look ‘bad.’ Now that most people no longer use Classic, and Jonathan Ive is into solid-colored hardware, the stripes no longer serve any purpose.

Even stronger than that; I think stripes were absolutely about making Classic look different and old, and now that you don’t get Classic unless you go and hunt out a CD with it on (was it Tiger that dropped it, or Panther?), you won’t notice how much easier the Classic windows were to navigate, drag around and so on. Personally, I keep ‘falling off’ the edges of windows when I’m dragging them – which I never used to do with OS9 and earlier.