There are two main problems with requiring the CD to be in the machine to play:
1. The ease with which practically anyone now can burn 20 copies of his/her original SE4 disk.
2. There are times when it is convenient to not be required to have the CD in the machine to play. (i.e. multiplayer on a LAN against a few friends that haven't bought the game (but will as soon as they get a taste of how great the game is
))