Although it doesn't have a huge impact on play, so many buildings being invulnerable bothered me enough to mod it.
...and wow there's a lot of buildings I had to un-invuln. There's more invulnerable buildings than normal ones.
Buildings like the command building that gives you a general have to be invulnerable to keep you from being able to build and destroy it over and over for more generals, but if there were another solution to prevent a person from farming generals, you would use it, because invulnerable buildings are lame. A brain-offending implausibility. If you want a non-lame invulnerable building, create a precursor force field building or something. Other than that, the invulnerability should be used only when the game's lack of another way to prevent farming makes it necessary.