We tried using mac minis as little servers, but they wouldn't boot if a monitor is not connected. We got around it by making a dummy DVI connector with a resistor in it. But still, it is jumping through hoops. That and the silly BIOS, it is as if Apple went out of its way to make it less user friendly...
OS X client boots fine without a monitor attached, it just don't initialise a display device. You can connect via SSH and use other services. I think if you enable VNC in Sharing, it will simulate a 1024x768 display for you to use.
It is probably because your claim is easily disproven. I have a headless mini in my server room right now that has no problems booting. Turn on screen sharing and you can login and use it just as if you were sitting in front of a similar box with a display and keyboard.
Well, the generation we have doesn't. We just built some DVI connectors with a resistor on the back, to make it behave similar enough to a connected monitor.
Mac minis are fairly different depending on the generation.
