What about two external screens? What if your virtual X size is not big enough for two screens side by side but it would be big enough for one over the top of the other (I'm looking at you, fglrx)? What if I don't want do disable the internal display when closing the laptop for a short time?
What also wasn't shown: What about window placement? When I disable my external screen, everything is moved to the remaining screen. When I re-enable it, everything stays there...
But you definitely don't use more than one monitor. Because this bug is confirmed since 5 years for Multimonitor setups.
Multiple monitors on the same video card are hit and miss.
Multiple monitors over multiple video cards hasn't worked properly with most Linux desktops in years.
(For example, just install >1 video card, hook up a monitor to it, and even if xorg.conf is properly configured, it won't appear in Gnome / KDE GUI's to position monitors. Icon dragging between different X screens is a no go although it worked in CentOS 5.8.)
If you are putting in the effort to deal with multiple monitors, please consider the case of multiple video cards. (4 video cards X 6 monitors = 24 monitors is a reality in terms of hardware, it needs to be a reality in terms of software as well.)