Why would you think you need the CD in the drive?
Oh, yeah, because tons of stupid, idiotic, moronic, feckless, brainless, creativeless, corporate morons seem to think thats an effective method of copy control.
Thankfully, SD is more than halfway intelligent. Not only do you not need the CD in the drive, they don't care if you install it on multiple computers (that you own). Don't go around spreading your account to a half dozen friends, mind you, but if you want to play a LAN game up to 2 people can use any given CD key. (Not applicable on ICO).