Because I could use remote control with X11vnc server started at launch with/without monitor plugged to the PC. But when the monitor not plugged, the remote control is very slowwwwwwwww and can't be handy. When I installed and configured the x11 dummy video driver, the remote control went back to normal fast behavior, but it makes the computer stuck with no possibility to plug any real monitor. That's why I bought a dummy video DVIport dongle.
Stellarmate Os and Astroberry can do this without any hardware dongle, so I guess it is possible to handle this dynamically with code. I need to confess I have not spent lots of time on it, but I know it is possible.
At the end, you're right, the easiest way to handle this is a hardware dummy video dongle.