Just for the sake of it, I will explain one of the reason why griefing is usually not allowed.
It is because in addition to the normal building, you need to spend insane amount of time and resources into your machines and stuff.
If griefing between friends can be kept to a friendly competition, it is rarely the case with strangers, we usually don't care about them and strait off steal what they spent time and resources making.
I do agree that, if done the right way, with regulation and all, it can be fun, even more if the fighting is an all out war in a certain world, but all others are safe heavens.
You could have an accelerated world with many ores as the battleground, to give people more reasons to get into the fighting.
But most of the time, people would just destroy and steal, ad that is only fun for them, where they are making hours and hours of work vanish in minutes.