>(In the course of that same semi-shithammered discussion with the
>Chief, Don opined that the sphere must be some kind of engineering
>demonstration, because, he argued, any civilization technically
>advanced enough to construct one would long since have progressed past
>the point of having any practical use for it, to wit: the Time Lords
>don't even have one, and the Time Lords have one of feckin'
Well, if The Deadly Assassin is anything to go on, the Time Lords encased a Black Hole with their planet, and draw power from it, so they would say they did the same thing, only better (smaller, more powerful, takes more know-how).
(Then Rassilon would name the Dyson Sphere "the Sphere of Rassilon" because he was like that.)
Also, it depends on the alien race's manner of thinking and technological development. If they were some sort of Hive Mind that didn't work over interstellar distances, they may have "put all their eggs into one basket." Or thought they were shielding their solar system from Dark Matter Planetvores. Or they set Von Neumann machines going to make the Sphere, then died off before it finished and never sent a stop order.
("GIR! WHAT DID YOU DO?"
"I MADE A BOUNCY BALL!")
*Really, I could see the Irkens liking the idea.*
The Culture (mentioned elsewhere in this post) never bothered with much with Dyson Spheres, considering Orbitals (small Ringworlds) and GSVs (GINORMOUS space ships) to be sufficient.