We’ve been there before, and intermediate layers between the platform and the developer ultimately produces sub-standard apps and hinders the progress of the platform.
The second statement does have merit from the business perspective, as for example, allowance of third party cross-platform development tools can slow down widespread adoption of new features introduced by Apple. Third party tools may not be as ready to incorporate those new features as Apple would, especially if other targets cannot support them.
As for sub-standard apps, banning third party tools does little to address this issue. Many apps created with Apple's tools are absolutely terrible, but similarly bad apps in iPhone 4.0 will win a free pass simply because they were created with Apple's tools.