This shouldn't be done for two reasons:

 1. It won't support building on 10.7.
 1. It opts in to secure coding by default. Secure coding should be explicitly opt-in, because it requires conscious thought on the part of the implementor to make sure that the behavior is _actually_ secure. Even if the default behaviors of `MTLModel` are sufficient, we need the implementor to sign off on that by explicitly indicating conformance.