... pointless once you've found most of the locations in the game and can fast travel to them.
Not if time was an issue. That's one of the chief disappointments that I have with FO3 & NV, is that unlike the rest of the series, these games are basically timeless.
No, map is too small. It worked in Fallout 2 and Fallout Tactics because the maps covered multiple states, so vehicles were practical.
Wait a minute though... How often do any of us drive out of state? Most locations that I drive to are within 5 - 10 miles.
I see no problem (beyond the technical), of having a drivable vehicle in these games.
(In fact... I can show you what it could look like, though granted, this is not a polished and complete mod ~yet. http://www.youtube.com/watch?v=B7nxZUeAe0w )