From Jamie Kosoy, on the Big Spaceship blog, about the default Flash loader graphic:
The Beautiful Default is a visual trap – it’s something that looks “good enough” to be deployed as is without modifying it’s out-of-the-box look. One the one hand the Beautiful Default allows you to create beautiful things quickly and easily, which is awesome! On the other hand it also makes the thing you’re making look generic. It’s vanilla. It’s the same thing everybody else is making and doing. Think about that, and then think about the majority of iOS apps you’ve seen. Think about all the websites you’ve seen that use Lightbox or Isotope. There’s a sameness to them that’s comforting and easy to use, but there are also opportunities to be expressive and interesting that are lost. There are too many developers and designers giving up on craftsmanship to build “good enough” sites and apps.
I kind of disagree with that last point – iOS’s beautiful defaults have pulled the lower bar of quality up to a level significantly above “good enough,” and even above the level that most designers can produce. The vast majority of app developers and designers are simply not talented enough to produce interfaces better than the iOS default, so if they attempted “good craftsmanship” by customization, they’d actually make their products worse.