Don't think, that loosing your spaceport will happen so often, that this would made sense. Also, when you rebuild it in exact the same way, it will be a lot more expensive, as the step-by-step way. (You still must pay the price for the components.) And when you are in a war, were you loose a great number of ports, I'm quit sure you'll have more important things in mind for your minerals, for example building a fleet in order to stop those bastards destroying your spaceports...
And, last but not least, there's still a chance, that you'd prefer your new spaceport to be tailored to your actual tech an fleet plans, instead of the old ones you designed 50 or more years ago...