Like you say, you can TNT proof it if you have the resources, thus making it impervious anyway. TNT just means that you CAN be robbed while you're offline if someone has the resources to make the TNT, and thus gives you the incentive to make your base harder to get into with that regard (thicker walls so someone can't just get one TNT and blast in or obsidian lining)
This also due to the larger amount of resources required to be defensible gives people a reason to band together into actual factions
For perspective on TNT rarity as well, I would personally have nerfed creeper spawnrate into the ground so gunpowder would be rarer. I do this not to make gunpowder rare, but actually because 90% of the time there is as many creepers as all other mobs combined, which pisses me right off since a) they don't burn in sunlight and b) they cause actual damage to the stuff you've built. Trying to farm gunpowder would be an exercise in futility.
Poll