Well, I do the same, but with a retail version of Windows 7 Ultimate. My reason is this:
Have got some nasty keyloggers in the past, so now I Ghost my PC every friday, by turning on my eSATA-HD, and let it boot from eSATA-drive, make a ghost image of my daily Windows 7, onto the eSATA. Shutting eSATA down, taking the power from eSATA, and boot my PC normally. This way, if I got a virus, malware, keylogger or anything else, I can be sure, that it is NOT on my eSATA, as it is only turned on while ghosting, and the netcard is deactivated, so nothing comes into the backup. Therefore I can always restore back to a 100% virusfree installation.
But yes, this requires 2 installations and 2 activations that will look different, as it is done on 2 different harddrives, so Iøm not sure about an OEM, but my retail-version did it without a problem. I think it is legal to do this.