For a triplehead setup this concept is just not bearable. I think this really bad design-decision goes back to the days when there was this "Linux the great netbook / eee-PC OS hype".
The good thing is: Nobody is forced to use this . There is Mate (http://mate-desktop.org/), or KDE, or XFCE, or ...
The only thing that I find a bit pity, is that so many resources are spent on Gnome3 and Unity :-/
Most have split their desktops into 9 spaces and run all apps full-screen, usually with a terminal on the notebook screen and something else (Eclipse, Emacs) on the larger LCD.
It is my biggest gripe with OSX that the "Spaces" functionality requires multiple clicks with lengthy animations in between to navigate the spaces. I could setup keyboard shortcuts but, damn it, a one click panel or dock widget is what I am used to and what I like. This has been included in unix type IDEs since the early 1990s (except Apple who managed to break it) the lack of it is a deal breaker for me.
Do we get one in unity.
On the Mac, I like the Lion way to deal with it, despite it being unidimensional. The only thing is that you'll need to get rid of the mouse and buy a trackpad (assuming you are on a desktop or with the notebook docked).
I find it confusing when both heads move independently between their own spaces.