That is a good point to emphasize. A downside of a CLA is that it adds a bit of bureaucracy and may deter some contributors. If the primary concern is whether a GPL licensed app is publishable on an App Store, an alternative is to add an app store exception clause to the license. (The GPL allows optional extra clauses to make the license more permissive.) Though this means that while your code can be incorporated to other GPL licensed applications, you can’t take code from other GPL projects that don’t have the same exception.