It may not be piracy per se, but it clearly breaches the EULA and is forbidden.
As far as I know, it also does not work very well. SI have coded the game with some nasty surprises, should it detect that you are using anything other than a genuine paid-for copy.
It's really not a great hardship to move the CD from one machine to the other, is it? I can however vouch for that feeling that you get when you switch on your laptop and fire up the game, to be told to insert the game CD, only to realise that it's a couple of hundred miles away...
You could always buy a second copy of the game. You can pick 'em up quite cheap if you shop around.