On reflection, the rendering engine in EW has to do much more than a browsers. It has to allow users to move items, resize them, their borders and margins, etc. Its probably easier to write from scratch rather than adapting IE's engine, but it would still be nice if it rendered at bit more accurately!
I hear you, but when you are learning CSS, EW can convince you there are errors in your code when there aren't! I spent ages looking for the problem in some simple CSS for a row of tabs, only to find they worked perfectly in IE, FF and Opera and passed the W3C test.
I still find FP2003 renders more reliably than EW, I'm sure this is something MS will want to improve on in later versions. IMO its the one thing making EW a good tool not a great tool!