It does work, just not while in the steam common folder.
Just copy and put the entire d3 folder into a new location (I did C:\Games\Descent 3 for ease of use). Then all you have to do is update the new d3 to 1.4 patch located in this thread http://dbb.d3k.org/viewtopic.php?f=1&t=4953 and make sure you have the mission. use the .bat file to join and you're back in the game. I would not recommend using .exe file from d3.descent.cx, as it caused a re-occurring memory dump for me. The .bat file from there should be ok though. If you don't create a copy in a new area Steam will most likely do a file repair and override your patch.
For some reason I wasn't able to get the 1.5 patch working. Perhaps the servers don't like it.
And for those people with questions about steam changes, it's almost entirely the same as the cd-version, the only difference is that ability to join a friend's game through steam.
So go on and buy away if steam is your thing, gog is still there too.