Methods are often added to core classes like NSString. For example, in iOS 6: https://developer.apple.com/library/ios/releasenotes/General...

And you changing the category won't help the versions already out in the wild. Hopefully, automatic updates will mitigate this to some degree but not everyone will turn them on.

There are workarounds (like prefixing methods with a pseudo-namespace), but they're all a bit ugly. Might be worth considering though.