Sorry, but that's because it pretty much is, they never have an actual argument behind their rhetoric besides some excuse about how this toughens people up or something
I ran the NCR and Yes Man endings last night and in the latter case chose to talk Lanius down, and at one point he mentions how letting the war happen would be a good thing for the NCR because "..it would make them stronger by culling the weak and leaving the strong to continue fighting". This and a comment he makes in his war tent if you choose to back the Legion make it pretty clear that all the Legion cares about is if you are strong and willing enough to fight on their behalf, and if you aren't, then you are either enslaved or killed.
Interestingly enough, you can prod some of the leaders about the slavery and they admit/suggest (it's hard to tell) they don't really care for it, however they go on to say that it is necessary for the strength of the Legion and then refuse to discuss the matter further. A couple of them hint that they don't exactly agree with Caesar's policies, but if you press them on the matter they get all bombastic about 'doing the will of Caesar', and I chose not to press further as doing so would probably have provoked hostilities, which I did not want on that character.
While I doubt I will back the Legion again since they make me take out the BoS, whom I like, I will say that if you are going to you should act accordingly from the start, rather than doing as I did and using a save I'd made prior to choosing a side (I was grinding completion achievements), as that felt unsatisfactory to me after I was done due to some of the end comments not making sense relative to the outcome.
To do this, you start by chatting up Vulpes Inculta in Nipton, and then heading for Cottonwood Cove where you can get some work as well as enable the submitting of NCR dogtags for cash and Legion fame. There won't be much to do for the Legion quest-wise unless/until you advance the MQ, but there are some NCR quests you can work in the Legion's favor and if you are careful about it you can take out the NCR's forward bases without losing rep with them. Of course you might
want to lose rep with the NCR, but they do offer the most quests so it might be wise to at least keep them neutral for a while. Since the Legion despises gambling you would want to avoid The Strip as much as possible, only going there if a quest requires doing so. Any quest involving a lesser faction's alliance should be worked out in favor of the Legion whenever possible, and if they refuse, expunge them.
I had planned to do this with a Melee/Unarmed character, since that's how the Legion fights, but the end quests soured me on backing the Legion again, so that character will go the Independent route instead as it has the most flexible options in terms of what you do with/to a given faction.