FIRST. Yes, This is a duplicate. I attempted to have my reasoning heard on the issue that is already available but it has been ignored. Please take some time to hear reasoning from a Mac user and realize that the Quit function needs to be implemented. I am going to provide some backing to my reasoning using Apple's official OS X Human Interface Guidlines (HIGs).

Every single Mac app, according to the HIGs, has an "App Menu", which is the menubar item that says "Minecraft" in this case. This menu is expected to contain "About AppName", "Preferences...", "Services", "Hide AppName", "Hide Others", "Show All", and "Quit AppName" (1). While Minecraft has all of these (minus Preferences but that's reasonable), the Quit menu does not function.
The Quit function also appears in the HIGs' list of Standard Keyboard shortcuts. If you scroll down to the letter Q, you will see that every Mac app is expected to use Command-Q to quit (2).

It has also been made clear that the reason that the Quit function does not work is that people believe that the OS simply stops the process. This is not the case whatsoever. As seen in the screen recording on source 3, the Quit function does not stop the process. The example I used to create the screen recording is saving a document. If the OS stopped the process when the Quit function was executed, I would not be prompted to save the file.
These are system-wide functions that all Mac developers are expected to follow and, therefore, this should be added to the Minecraft Mac client.

Thank you for taking the time to read this essay

Sources:
1. http://developer.apple.com/library/mac/#documentation/UserExperience/Conceptual/AppleHIGuidelines/Menus/Menus.html#//apple_ref/doc/uid/TP30000356-CIHDGFJB
2. http://developer.apple.com/library/mac/#documentation/UserExperience/Conceptual/AppleHIGuidelines/KeyboardShortcuts/KeyboardShortcuts.html
3. http://youtu.be/45GDEclqQNg