I don't see why not considering you have already purchased it since you have a valid CD-Key. But I can see people making the argument that it isn't any different then just stealing the game/product.
For Diablo 2 I had lost the disc's, but had it already installed with a verified CD key from like 2 years before so I downloaded the disc's I needed to play it. I see no problem with people doing that.
This is something that a current debate in copyright law right now. The whole "if i have a legit copy of a game why do i not have the right to make a personal copy of said game/media ect." Right now as it stands I believe music and movies have been cleared up that you can have a back up copy of a movie/music for instances like "im going on a trip and I don't want to take the original." But the realm of games is still foggy.
If you are interested in keeping up with how this stuff is playing out check out http://www.eff.org/
I always buy my games for the added value of online play. I store my CD key in my GMail and either torrent an image or rip an image myself and store it on my PC then lose the disc.