as to the first: as other have outlined in other threads, you get two abilties with the meteor upgrade, 1) the ability to attack planets and 2) the ability to attack ships. in most cases you'd primarily go for it because of the 2nd one, but hey, you get the first one also and it may be useful if you are attacking planets. it means that you will need less/no siege frigs, which with the price and all is always a good thing. so, in short, no, the meteor upgrades are also quite useful in defense, just not the planet attacking part of it (except that the planet gets nuked and colonised by the enemy while the starbase is still alive, however likely that is).
as to your seconds remark: as a matter of fact they already are. if you compare roids with planets, they start out with 1000 infrastructure instead of 1.500 and can only have a maximum of 2.500 versus a planet's 6.000. granted, it's not siege ship related, but as is, asteroids are more vulnerable to sieging that vanilla planets are.