This is an awful argument.
Those who are going to pirate apps have the means and skills to do so. Even on the precious iPhone. Yes, you are able to pirate Android apps quite easily as a lay person.
An intermediate approach would be a system where binaries distributed exclusively via the App Store could (enforced using the same kind of signature system presently used) only be installed via the App Store.
Give developers a choice rather than making it for them.