Just installed VisualDMX today, and was surprised by the very interesting features.
A good decision to give away the software for free, of course. Most lighting desks come bundled with software which only works on their specific desk, while 'older' PC's are constantly thrown away. A PC with a few fader controls also makes a good lighting desk!
Since your main concern seems to be the hardware, I can perfectly grasp the idea of VisualDMX only 'working' on the Visual Productions DMX dongles. Which is a good decision, since it encourages users to purchase your products.
You do realize, however, that adding this server functionality opens the door to a whole new area, and possibly enabling clever third parties to develop their own solutions of allowing DMX output without the VisualDMX hardware? Having the backing of the community might also be a good thing in this, though. I'm not sure if you as Visual Productions would be happy with people doing this. Primarily the smaller market segments (prosumer, enthousiast, small theatres/stages) might go this route.
It also opens up another few interesting areas of control, though, as the server would also allow uplinking with VJ packages (e.g. Resolume), DJ software (Traktor) and all kinds of software sampling and synthesis applications (like Ableton and Reaktor) via Open Sound Control. Which would make for a completely interactive, artist-driven performance... think in terms of Jarre (Europe Tour 1993) with him controlling everything from his big keyboard. Hitting a note or a chord migth trigger an array of presets and effects, including the video, lighting and laser environment. This would effectively turn the artist into a DJ, VJ and LJ at the same time, and put him/her at the center of attention.