In Windows Vista, when Aero transparency is enabled, you may have noticed that this transparency goes away when you maximize the window. Many users wanted this transparency effect even when the windows were maximized. Some even went to the extent of suggesting that Microsoft, in its rush to release Windows Vista, had actually forgotten to give the maximized windows this effect. Many used 3rd party apps to achieve this.
Microsoft was at pains to tell that this was actually performance optimization:
“Opaque title bars are more efficient than translucent ones, and when you maximize a window, you’re saying, “I want to focus entirely on this window and no other windows really matter to me right now.” In that case, the desktop window manager doesn’t bother with translucency because you’re not paying any attention to it anyway.
This may seem like a very minor change, but the difference is noticeable on benchmarks, and, like it or not, magazine writers like to use benchmarks as an “objective” way of determining how good a product is. The reviewers choose the game, and we are forced to play it.” MSDN Blogs.
In Windows 7, you now notice that even maximized windows have this transparency effect.
What then happened to the ‘performance optimization’ explanation, in Windows 7 !? 
Hence my question :
Why is transparency now retained, even when windows are maximized in Windows 7 ?
