In short, the end justifies the means, and I believe that under Legion rule, the wasteland would eventually become a working civilisation, and surely if the end result was good for everyone, the people under Legion rule and the Legion itself, then do the means they use to achieve it really matter? I don't think so.
My reason's for supporting the Legion are as follows:
- A Pax Romana, as said by Caesar. Yes, the peace the Legion would bring would be built from slavery and brutal tactics, but there would be peace, and for the occupants of Legion territory, safety. Dale Barton, the trader in THE Fort states that it is more profitable to stick to Arizona as unlike the NCR, you don't have to hire protection, pay taxes for transport and trade et cetera, but in Arizona, traders aren't attacked by raiders, as the Legion acts as a deterrence, so trade would run much smoother if they aren't constantly being preyed upon by Raiders.
- Powerful army. The Legion is brutally efficient, as although they're methods might be a little extreme, their soldiers are un-wavering, willing to slit their own throats instead of being captured. Why does this support them? Because you've got an force that although uses low-end weaponry, is brutal and is committed to the cause they would be fighting for.
- Against materialism, which can bring about corruption and general dislike in the populace. NCR is an example of this, bleeding their citizens for money and taxes leading citizens to dislike the NCR and try to settle elsewhere.
- They try to craft a civilisation and at the very end craft a united society, and as shown by the game ending slides, (if you side with the Legion that is) it says:
- Yes, they enslaved part of the population, and I'm not supporting slavery here, but slaves can be useful. Yes it's horrid, but for construction purposes, a task force can be brilliant for swift construction among other purposes, but in the end. There would be civilisation, a working one.