You know, I've always wondered why you would actually blow a planet up, I mean that's pretty energy-inefficent isn't it? Surely long before you built up enough power to blow the planet apart, you could just superheat the atmosphere. That would kill every living thing on the planet and most likely render it uninhabitable, problem solved.
In terms of the game I think it would be nice to have Terror Stars, but only if they are balanced. One possiblity would be to make the main cannon for it ridiculously costly to research. I think terror Stars have potential as logistics / support facilities even without a planet-buster on board.