A good thing for a platform is for applications to have consistent look-and-feel. Not just because consistency is good, but because the look-and-feel should also be a good one - one that is usable and exploits all the platform has to offer.
While style guides are helpful as references, a far more powerful tool are flagship applications. Such as MS-Word and IE in Windows or iTunes and Safari on Mac/OSX.
Apple used to know this, but they are now trademarking their own bubbles!
And so instead of using their apps as demonstrators, they have gone 180 and are now actively blocking people from using the same UI elements. So each developer must go and make their own unique bubble, leading to wasted effort - of the kind that a platform should be preventing - and, moreover, guaranteeing a messier user experience.