We believe that taking something complex and adding a familiar, declarative syntax, while keeping it all 100% native, will appeal to both developers and designers. We believe so because we've asked many of them.
Animation of custom shapes seems like it would get into <canvas> territory. The canvas drawing API shares many commonalities with CoreGraphics, and I assume that CoreGragphics played some part in its design. A web developer should have no trouble transitioning to CoreGraphics (which, I will add, is not an Objective-C framework) in such a case.