advantages of 3D models for planets?? explosions man!! special big ass beam cannon or something, fire it at the planet and BOOM! instant cataclysm. maybe have "cities" mapped on it somewhere if you like, little bit of quick math, BOOM! XYZ coords for subsystems. maybe have "live" subsystems that you can blow off, like shoot new york and have a HUGE chunk float off into space. planet based orbital defense systems, massive turrets based on the planet (nothing much really, since you likely wouldn't get close enough to see them) with long ranges so they can Reach Out And Touch You if you get too close

whatever you want, or something

also, massive planets, like 12756000m wide (earth size) don't slow down the game too terribly, even when they're exploding, I've done this already

just as long as you keep the poly count down, I was around 1280, I believe.
(sorry, operating on a LOT of caffeine right now, pardon the glaring punctuation mistakes)
Bob
Idealist