Its not a feature, its an implementation artifact of the window frame being drawn by a separate process than the window contents. Its broken by design. OS X does it the right way, with the window contents and window frame being redrawn in lockstep and double-buffered so the user never sees partially drawn windows.
This is probably a legacy of X's network oriented history. If your window contents are being rendered by another computer on the end of a high latency connection you want to be able to reorganise your windows on screen without waiting for the other end to finish the reflow.
I assume that window contents are drawn to a buffer before being rendered on a modern system anyway. I can't remember the last time I saw partially drawn window contents.
Well, even if you assume the app will take too long to render, is it really worthwhile to render the "rest" of the window (pointer, frames, etc). The main reason you're working with a particular window is the content, not the chrome, so drawing the chrome asynchronously only serves to make things look weird, not actually get things done faster.
