The difference is a sense of accomplishment. I can't stand having that big major battle hanging over my head while I'm out picking flowers, instead....
The thing about it is that the battle isn't really ganna start unless someone made the first move. Its giant chess game. Apprentally, the NCR started the first move.
Spoiler When the NCR's President came and visit the Dam and expose himself to the open, this bring the Legion to commit immediate action. Fail or success would cause an all out battle.
Other than that, there no way someone ganna generate a big battle unless something important happen.
Again, why are people so opposed the the option of being able to play after the main quest with no real changes? I doubt it would be hard to do and if you don't want to utilize such a feature, don't do it. What is the reasoning behind opposing such a basic option?
The ending was meant to show the meaning of all the action the PC has done. Adding a "end" game part would not really put much an impact to the ending itself. Bethesda was able to do away with it due to..
A) Its their habit in their making of their game and
B ) The faction in Fallout 3 is more lax as ya already an "honorary" member of the Lyon's Pride.
Beside, the only thing gain from that battle is that helm and that sword, seen as "trophy" but other than that, worthless.