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Why in the world do you care if I can or can not add items to the menu? How does it hurt you for me to have that ability? It doesn't. So why are you arguing against it?
Because it adds a button. Gnome is about removing preferences and ui elements. Simplify, simplify, simplify. Computers are complicated enough.
Applications should create .desktop-files. If it doesn't upstream, the distro developers should do it. Many distros are always happy if someone files a bug about missing desktop files.
And if you install something from source, you're knowledgeable enough to get smeg. Or, again, the distro developers can include it by default, as ubuntu does if I'm not mistaken.