Hello.
I can understand that people that have a solution like Milestone and have the need to record monitors use the ONVIF solution to record Desktops, but as said this is a great solution for shops using POS systems.
It dates the images with the surveillance images and in case of fraud, theft or other issues that need to be looked at the screen recordings can be seen at the same time as the recorded camera images.
another subject could be a limited budget small shops don't want to spend more money on licenses, manpower and a server to record this separately.
Now, if you are a cooperation that needs to record calls and screens at the same time (Like many Support companies do) i agree with you that a dedicated software solution is the better solution.
But not the ones you named
IP-GUARD isn't a very professional solution for big companies to much bells and whistles to that software if you just want to record a screen.
And i think the same of SurveilStar, to much additional stuff noone needs as most companies have all in one security application software in use that uses policies to avoid visiting unwanted websites and other things.