The way it's demonstrated on the website, there's no visual indication that the window is partially offscreen except for the lack of a rounded corner, which isn't noticeable at a glance. The exact offset would have to be played with a bit, but letting the buttons push slightly off the screen makes it immediately clear that part of the window isn't visible.

Another visual indication you lose either way is being able to judge how much of a window is off-screen. Normally, the title is perfectly centered all the time. You can look at how far the window extends to the right, and assume it does the same on the left. My minor edit to the buttons above doesn't help with this one.

All in all, I think it's giving relatively minor gains in exchange for removing visual information about the size of the window. The effect is similar to Apple's choice to hide scrollbars in 10.7 unless you're actively scrolling. Sometimes you can judge from the context that there's more available off the screen, and sometimes you can't. Is it worth the trade-off?