I dont see the problem with letting you explore the mojave after the games complete. I have usually explored everything by the time ive completed it so it doesn't really bother me. But if it keeps some people happy then why not. The ones that want the the game to end after the slides can just start a new character. simple
It's not quite that simple given the large number of variations the New Vegas ending can have depending on your actions up to the end of the main quest. If they just let those who wanted to keep playing do so without changing the gameworld to reflect that players actions, I'm pretty sure there would be a lot of the people wanting continued gameplay after the main quest on the forums raging because the post ending gameplay does not reflect their choices in the game.
I'm reasonably sure that Obsidian decided against post ending gameplay for several reasons one of which I think is that with the timescale they had to work with they knew they could not take into account the characters actions throughout the game and thus couldn't create a 'seamless' post ending gameworld. Also the game would have to keep track of the various ending segments you can possibly get, of which there are a total of 27 different segments you can unlock, although one of them is negated by one of the others, so there are a total of 26 ending segments you can see at the end of the game. On top of this each of those 27 segments has anywhere between 4 and 13 variations to them. Trying to keep track of all that would increase the games programming complexity by at least an order of magnitude and on top of that the more complex a program is the greater the chance of more bugs and glitches.
@WestoftheRockies. The Vanilla version of Fallout 3 had a definate ending so your example is incorrect citing it as a free roam style game. Mass Effect 2 while it may let you continue playing after the end of the game, it's post ending gameplay leaves a LOT to be desired.