I made two dummy plugs today
Dummy plugs have no use as of the last few driver 1x. versions in windows. Not sure about other OS.
Yes they do ; )
1) Windows Server 2008, clean install, any driver version - no GPUs work at all without dummy.
2) Asrock 970 extreme3 r2.0, Windows 7, my friend was having issues until he plugged dummy in one of the cards, can't provide details atm.
But display connected/dummy forces Windows to make that GPU primary/master and sometimes it matters.
3) nVidia + AMD cards, Windows 7, unless dumy is plugged into AMD card it won't work