Holy cow, I wrote a book!
The taskbar created all sorts of interesting problems, since the
work area was not equal to the entire screen dimensions.
(Multiple monitors created similar problems.)
"Why didn't the gui return the usable workspace
as the root window (excluding the taskbar)?"
That would have made things even worse.
Lots of programs want to cover the entire screen.
Games, for example, are very keen on covering the entire screen.
Slideshow programs also want to cover the entire screen.
(This includes both slideshows for digital pictures as well as
Screen savers of course must cover the entire screen.
If the desktop window didn't include the taskbar, then those
programs would leave a taskbar visible while they did their thing.
This is particularly dangerous for screen savers, since a user
could just click on the taskbar to switch to another program
without going through the screen saver's password lock!
And if the taskbar were docked at the top or left edge of the screen,
this would have resulted in the desktop window not beginning at
coordinates (0,0), which would no doubt have caused widespread havoc.
(Alternatively, one could have changed
the coordinate system so that (0, 0) was no longer the
top left corner of the screen, but that would have broken so many
programs it wouldn't have been funny.)
[Raymond is currently on vacation; this message was pre-recorded.]