I keep feeling like the Legion is pretty underrated in terms of the discussion around the game thus far. Granted, in terms of quests there aren't nearly as many as the NCR has to offer, but as far as setting up an ideological counterpoint to the NCR I think they more than serve their purpose.
Sort of spoilers follow?
As I understand it, Caesar aims for a Hegelian synthesis. I haven't gotten the dialog, but the way it's been explained to me, Caesar surmises that by conquering the NCR, the Legion will gain its strengths but not its weaknesses - the rank brutality of the Legion will be tempered by the NCR's liberal Republican ideals, but unlike the NCR the Legion will be efficient, functional, and capable of holding territory and ensuring its safety (many characters, including Raul and the Arizonan / New Mexican trader at the Fort, emphasize how exceptionally safe Legion interior territories are). Thus the NCR is supposed to be the Greece to the Legion's Rome. In this, the Legion is principled in a way that the NCR isn't - for all its high-minded ideals, none of the NCR brass seem terribly interested in living up to their promises so much as they are in building a territorial empire. Caesar's an idealist - he wants to purify the NCR, and it's hard to argue against the notion that the NCR needs purifying. Nearly everyone you meet in a position of power in the NCR (above the Rangers, who all seem fairly sensible but at the end of the day are soldiers under orders) is petty, shortsighted, or buffoonish. In the cases of the McCarran scientist and Fantastic at HELIOS One, you see how careerist dickheads ignore reason and take credit for other people's work and are rewarded for it. The top brass are cronies of the President, who himself is a stereotypical politician, advocating terrible policies and carelessly throwing the grunts of his army into the meat grinder while he goes around making treacly, pointless speeches and currying favor with citizens who don't know or care how fucked up shit is (Mr. House pegs the NCR as being primarily concerned with comfort and indulgence, and if anyone should know it's him). The Legion encroaches heavily into NCR territory and they don't even seem to notice. Such dysfunction in the Legion would result in crucifixion or worse, in a very short amount of time. Playing the game for the first time you get the feeling that the NCR is hopelessly outmatched unless you personally carry them to victory, at which point they will promptly forget about you and continue to gleefully make a mess of everything they touch. What the Legion is offering is boundless sadism in the short term, and an ideal society in the long term.
(Definitely spoilers here!)
Before I knew about Caesar's plan (if you can call it that), the Speech victory you can pull over on Legate Lanius when not fighting on behalf of the Legion seemed sort of abrupt and nonsensical, but in the proper context it makes perfect sense - you essentially challenge the dialectic and convince Lanius that contrary to what Caesar believes / believed, conquering the NCR will corrupt the Legion and all the systemic problems that the NCR faces will persist in the new order. So Lanius goes back to Arizona until such a time when the Legion is truly capable of bringing functional civilization to the wastes.
SPOILER END
The character of Caesar is written by John Gonzalez but the dialectical reasoning is apparently all Sawyer's work.