Anything I use in my apps has to be open source in case either I come across limitations or support is dropped. I can't create a dependency in my app that could severely cripple me in the future.
It seems like using Pixate would introduce a huge dependency on your application and everything that Pixate does is mapping down to something already done in UIKit or perhaps CoreGraphics. I can understand the allure of this framework because it would save you writing a lot of code yourself but when you come across a limitation (which you undoubtably will because not even the mighty UIKit does everything we need) and you implemented it yourself you can then easily make improvements rather than waiting for a third-party. Sure you could make requests but if Apple honoured every single improvement request on UIKit they would never ship.
As an iOS developer I'm already in bed with Apple but their track record with AppKit and UIKit speaks for itself. Their frameworks are black boxes but I can build upon the shoulders of giants. Third-party UI frameworks like this feel like I am constraining myself to very explicitly defined limitations and the second I cannot adhere to those limitations I must be the one to compromise instead of striking out on my own.
Is there something that I am missing with these third-party UI frameworks because the opportunity cost has always seem to be too high for me.