I can understand LUK's hesitation to make the project open-source. Once open source he'd lose control, because instead of having a hardworking individual working on implementing truly useful feature additions, and working on performance and stability of the program, the development would turn into something like this:
1) Even though it deals on a hardware and low level OS level, people would want the project to be ported to the three major OS streams (Mac, PC, Linux). As a result of this, Everything would have to use *nix widgets that just look ugly in Windows, and every window would be a grotesque waste of space.
2) In trying to make it more interoperable between OS's, the program would run using something that minimizes change of code base between OS's. As a result it would either be a big messy Java beast, or it would force Windows users to install Cygwin (so much for a 1MB burner software). In any case, support would have to be dropped for anything lower than Windows 2000.
3) In all this mess of porting code base to different development platform the program will be extremely buggy and lacking in any functional updates. The new project developers will start begging for additional developers.
4) Instead of doing something useful like improving the stability and functionality of the program, all the new developers will be busy making skins and useless plugins. So we will now have Hello Kitty ImgBurn, and the ability to video-conference in ImgBurn, but it won't work on half the Burners out there.
Really it's no great loss to me that it isn't open source. One major "benefit" cited for open source is frequent updates. In my experience these updates are required because the program is really crappy, full of bugs and needs updates for functionality reasons. LUK is actively developing the program and bugs are minimal at best. Even if development stops it won't matter because the program does what it's designed to extremely well. Another example is DVD Shrink. Although development has stopped, it is still a great re-authoring tool and will remain that way.
I've frequently found this to be the case, where closed sourced freeware vendors can provide software far exceeding the quality of open source efforts.
If you're interested in Open Source Disc burning, look at InfraRecorder.