Seen this topic crop up several times now and it actually caused me to stop playing my first character, which was a Legion supporter.
The fun of the game has practically been sapped.
It is my strong opinion that the game should NOT have advertised the Legion as a major player-supportable faction. For that matter, there were some pretty bad design decisions made as it's quite easy to land your character into hot water very early on with the NCR, with no way to "repair" the game experience or continue on -- an example is if you stupidly decide you want to play a rebel-type PC, and then support the Powder Gangers. Three "go do this one thing right outside the door" type quests is not worth backing the group, since later it doesn't provide you with ANY sort of foot-in-the-door scenario into winning the Legion's hearts or branching off into other quest paths, but it DOES earn you the privilege of being continuously harassed by higher-level and much-better equipped hordes of NCR rangers while you're still level 6.
If their intent was to provide a player with the excuse to go on a murderous rampage and loot everything on Earth, then I am very disappointed in Obsidian. You can do that sort of crap even if you back the NCR or go independent. It's not hard to make Legion quests that are morally ambiguous or would help color the way you see them and their more average-type people -- an example of what I'd consider to be a SINGLE decent questline was the Brotherhood faction from Oblivion. Dark, shady, but fun and immersive.
That said, there is no reason on God's green earth that all the people in the middle of the crossfire (all those minitowns, Goodsprings, Primm, Novac, etc) should whole-heartedly back the NCR. It's made pretty clear at the start they want safety and to be left alone. At best, these areas should be NEUTRAL, and unwilling to go bat[censored] insane on you if you decided to back the Legion but were cordial enough to bring them trade, kill the ebil supermutants om-nom-noming their battle cattle, and run fetch missions for them.
They made NCR out to be good guys, but they are deceptively so. BOTH groups would draft people from their populations into armies, take supplies, and set up camp. If anything, I would think that the Legion would be a BETTER choice if gone into willingly -- at least they're not incompetent.
The problem is
NOT that the area is NCR dominant.
The problem is that there is VERY LITTLE gameplay for the Legion.
If you only had 15% of the map be friendly towards you, but you had 50 hours of missions and an entirely in-depth, new way to approach new areas OTHER than "KILL KILL KILL KILL KILL LOOT LOOT LOOT" then playing as the Legion would be successful and fun. But it's not. It's a tacked on faction that beckons you to back it because it's sixy and a lot more interesting culturally than your average NCR "d00d" or moronic outlaw, but when you do, you find there's nothing to do for them and you've only managed to shoot yourself in the foot.
I will come out and say it. I think that making out that the NCR is all sunshine and puppies and out for the good of everyone in the wasteland is brainwashing pro-American bullcrap.

It's like a public high school history book shoved into a video game.