Option-clicking the green button (usually) maximizes. Full screen in a separate desktop space may resemble maximizing in some ways, but it significantly changes the interaction model when switching between windows, especially when some of the windows are not maximized. Example:

1. Open two apps, at least one of which supporting fullscreen.

2. Open three windows in the app that supports fullscreen.

3. Fullscreen one window.

4. Cmd-tab to the other app.

5. Cmd-tab back to the previous app.

If your window from step 3 was maximized (filling the screen in the same desktop space), you would return to that window, but instead you are now looking at some completely random other window that you were not interacting with at all.

For people who cmd-tab (and cmd-`) frequently to switch between apps, this is super jarring and not at all what we intended to do.