1. Different apps will draw decorations differently, leading to a lot inconsistency. 2. It means apps designed for other environments that don't draw their own decorations won't have any decorations. 3. If your client doesn't use one of the big all encomposing gui toolkits (not every app needs that), then it has to implement the decorations itself, which is non-trivial to do correctly (I know because I have worked on it for such an app). 4. You can't reliably configure the appearance and behavior of the decorations in a central place for all applications.
Fuethermore, if clients always draw their own decorations, then if you use a tiling wm and don't want decorations at all, you are stuck with them.
