Personally I have always preferred the model where there is one
encompassing program window, and then sub windows contained within it.
I can't stand millions of separate windows (i.e. GIMP). The taskbar
only shows one item for the program instead of one for each document
open. If the tabs you are talking about could be "minimized" (using the
term loosely) you could achieve the side-by-side behavior.
Here is a screen shot showing what I mean:
http://gbanaszk.oasis.nexus.carleton.ca/SummerOfCode/docsSideBySide.png