Yes, that makes planets much more resistant to bombardment and THAT is a good thing. Trouble is that the AI wont adjust for it so it will continue to ineffectually bombard worlds with its pea-shooters rather than invade them.
Actually, it wouldnt be TOO difficult to get them to research and deploy the Planetary Weaponry. But again, the trouble is that all the AI's ships would end up having them. The only other alternative would be to create specific 'classes' of enemy ships and have one of them carry planet busters, but there is no guarantee that the AI would ever use them appropriately.
So, in answer to your question, yes, some of us have thought about that.
If you are playing a multiplayer game, go for it and give it a try. If you are having AI players in the game, IMO, its not a good idea to mess with it.
Talenn