Caesar's Legion proves what the Romans and Mongols know: It's easy to suppress dissent in the outer districts when, if forced to act, you simply kill everybody. The NCR is unwilling to do that. So that means the NCR will find themselves in more difficulty in such scenarios, but it also means that the NCR is a nicer place to live in for the average citizen. It's not a tough decision for the people of the Mojave: The vast majority of them will die and a great swath of the survivors will be enslaved if the Legion runs through. This is not a controversial statement; merely an observation of what will happen. Meanwhile, the NCR already owns most of the region, and the only thing most people complain about are taxes. Well, nothing is certain but death and taxes and I prefer taxes.