Exactly. I would assume that anyone taking the effort to assault a planet would want to capture it for its income, special abilities etc. Destroying it completely (like with the Death Star) should, the way I play the game at least, be reserved for those times when the enemy is just too well dug in for you to be able to crush them.
"We think in language; therefore, the quality of our thoughts can only be as good as the quality of our language."
-- George Carlin