The assumption in the engine is that every frame starts with _clearColor in the buffer.

To see what happens if you don't do this, have a look at the last parts of this blogpost from my GSoC last year:

http://summermute2012.blogspot.it/2012/06/show-and-tell-in-10-steps.html

The point is that not every part of the dirty rect is dirty because we have drawn something new into it, but rather some of it can be dirty because we DIDN'T draw there this frame (but did so last frame).