There is no DRM on the Curse of Monkey Island disc. I just copied the files from both discs (it's a 2 disc set) and then played it using ScummVM. They have a tutorial on their site about which files are needed. No disc necessary.
Or there is a launcher that supposedly lets you play the game on 64bit systems without using ScummVM, but I haven't tried it yet. Not sure if it needs the disc or not. It's located here:
http://quick.mixnmojo.com/software/cmi-setup For Escape from Monkey Island, I haven't seen that for digital distribution either. I tried playing my disc the other day but it doesn't work on my pc. And I don't like the game enough to jump through hoops for it. But if you want to try the substitute launcher it's located here:
http://quick.mixnmojo.com/software/emi-launcher Edit: I would love for GOG to get the whole series on here someday. I'd definitely buy it just as a backup, and to get some of the "extras" like concept art and stuff.