(Edit: I always mix up the terms server side and client side decoration. Hopefully by my description its clear which one I'm referring to.)

Pro SSD: the window manager should decide how the apps look

• user: consistency across the operating system usage
• dev: generalized and less burden on the dev, less dependencies to worry about and development specific to GNOME
• both: flexible, change your window manager and the app changes without modifications (at least in ideal world)

At least for a normal window. I wouldn't mind if an application can request with a high priority that it wants to display its own decorations. In that case there should be an option to disable decorations from the window. That is how some applications do work and allows for a flexible usage based on users configuration. Now if there was just an official API from all window managers that can check this configuration, then the user wouldn't even need or know there is a setting for.