What is a good reason to tell clients who want their logo to be always included in every screen that it shouldn't be done for a mobile app?

1. What are the purpose of having these logos?
2. Why do companies want you to use their app?

What are the purpose of having these logos?

Logos are a point of identification; they're the symbol that customers use to recognize your brand. Ideally, you'll want people to instantly connect the sight of your logo with the memory of what your company does – and, more importantly, how it makes them feel.

Why do companies want you to use their app?

Advertisers. Advertisers prefer apps because they provide more information about a user's habits. This information is more valuable than the one which would have been obtained simply from browsing.