Hey, I do agree that you should be able to put honking big lasers and missile bases on your planet. Then, your planet could defend itself, or at least lend aid as a military starbase does, to the ships defending the world. That is an excellent suggestion, that has been made time after time, both in GC1, GC1 AP, and now GC2. Now, in GC2, you'd only do this on your military hard points, as plaentary tileage is just too precious to waste like that on safe worlds.
Now, let's just look at why we'd ever bother with building space ships (small and capital), if you had a mobile world.
#1) It would cost a lot less to go investigate whatever it is that you were interested in (ie, anomolies).
#2) Allows you to project force in mulitple places. For instance, you can park a few fighting craft near a resource point as well as have your forces attacking an enemy world at the same time.
#3) no disrupting the local planetary ecology.
#4) you can get mobile earlier (ie, at the start of the game, you cannot make an engine big enough to move your world).
Just off the top of my head. I'm sure we can find a few other reasons.