Fallout 3 is on exhibit at the Smithsonian. New Vegas is in the bargain bin at target.
Regardless of personal preference, I think we can all agree that the events of FO3/Broken Steel are fundamentally important to the future of the series and most likely foreshadow the events of FO4. As a Fallout fan FO3 is a must, NV is the optional proverbial cherry on top.
You say it as if Fallout 3 doesn't make it into "bargain bins" as well. To many, recognition be it from reviews or museums doesn't really play any factor in their opinion of the game.
Also, not necessarily. We don't know where the next Fallout will be set, in fact, I believe they told Obsidian to stay relatively away from Arizona because they had something they wanted to do there. In that case, if anything, New Vegas would be the one you'd want to play as a prerequisite. Either way, we just don't know. It's anyone's guess.
I do agree that it is worth it to get the game if you enjoyed New Vegas and are a Fallout fan, especially because it goes for low prices pretty much anywhere.