I think this entire discussion is comparing apples and oranges. To ask whether a backstory is important to a game or not is a bit like asking whether beer is "better" than wine, or which football team is best - it's entirely subjective.
Or to give a better example, which is better to watch on tv: A soccer match or an episode of "24"? Now, in both cases we're talking about a non-interactive form of entertainment taking place on a television screen, and which we (at least presumably) do not know the outcome of. That means there are enough similarities for us to compare them, right?
Well, that's the question, because I think what is being asked here is similar. There WAS a time when plot was important to computergames, because it was impossible to do one without a plot. Why? Because they were by definition solitary forms of entertainment with just one player. That is no longer the case in a world with virtual games like World of Warcraft (WoW), EVE online, Second Life (yes, I know they don't like being called a "game", but it's relevant to consider in this purpose) or the host of other MMORPGs out there. Do those need a plot? Not really... IIRC, WoW had more than 8 million players at the last count. That's more than the population of some countries, and with that many players, you really no longer need to enforce plot for the sake of entertainment, because that will come about simply by virtue of the interaction among the participants.
Case in point: http://www.break.com/index/leeroy.html
The point is, if you put just hundreds of people together, things will happen all by the themselves, and you don't need to paste a plot onto the game to make things more palatable and interesting.
In far too many games, the plot is little more than an excuse to drive things forward. Sometimes that's okay, and sometimes it's not. My best example of the that are the Star Trek: Armada games. Both are just glorified RTS-games set in the Star Trek world, where you build forces just the same as in Starcraft, Command & Conquer and hosts of others. The plot is nothing but a blatant excuse to make it all more attractive - it's pure icing on the cake. Does that work? Well, the reason I mention it is because it did for me in the first game, but not so much in the second.
Why? Because Star Trek is all about saving the starship, saving humanity, and saving the universe as we know it (and not necessarily in that order). Most Star Trek stories/plots are focused heavily on the individual people/characters involved, which doesn't exactly lend itself well to micromanaged RTS-game, where you often lose troops and equipment. This was acceptable in the first game, because losing a large Sovereign-class starship was actually a major concern.
In the second, however, you lost them by the dozen, and so the human factor was lost, because there was no way to win any scenario without sending large groups of ships, and so their crews, to their deaths knowing full well up front they would not survive. And, of course, building ships like that is not something done in an afternoon is not possible in the Star Trek universe, meaning that especially the second game strained credibility very far. In short, it was already strained in the first game, but I could accept it because (1) the story was good, and (2) the loss of starships and personel was not too unbelievable. But in the second game you lost troops and ships too fast to retain the human element of Star Trek. You had to calculate losses, which does not fit too well with Roddenberry's vision, since crews and ships because nothing more than disposable figures that you HAD to sacrifice to proceed in the game. And plotwise, they had Picard enter into an alliance with the Borg as a matter of course...
Now, I'm definitely a sucker for plot. Had I not been, I would never have played Armada 1, since RTS are not my cup of tea (the best Trek game ever, IMHO, is clearly Bridge Commander). Besides, my long years of playing tabletop RPGs probably makes me very predisposed towards plot. I remember when Magic: The Gathering came out and EVERBODY talking about how customizable collective card games would sweep the gaming market and end tabletop RPGs. I knew right then and there that would not happen, because the card games had no plot, and that was what attracted me to tabletop, and I probably wasn't alone there.
However, what I like and what is true are not the same thing. I may go for the plot every time, but that does not mean everybody else will. In fact, eight million people playing World of Warcraft would suggest that I'm a distinct minority in that regard.
As for the Asteroids game, it depends entirely on the set-up. If the plot is engrossing and closely connected to the actual gameplay, then the plot-driven game will definitely be liked by more people, I think. However, if the plot is a pure excuse for the game that serves no other purpose, then people will go for the plot-free version, I suspect. Just like the Star Trek: Armada games. We like plots, but not as mere excuses. And we MUST have something invested emotionally in the plot AND its characters if it is to have any impact on us as players.
I guess the conclusion is that games have evolved so far that there is no longer a simple answer to what you're trying to ask, SilentScope001. Are programs like World of Warcraft, Second Life, EVE online and all the other MMORPGs really games or communities? I'm not sure... KotOR is definitely a game, however, because you interact solely with imagined characters created exclusively for the plot by the developers to enhance the experience of the player. There is no question whether Atton or exile is the most important character in TSL. You cannot make the same claim about the character you play in WoW.
So it all depends entirely on what makes a "good" game. Which is, of course, completely subjective. "24" might be more dramatic than a soccer game, but the soccer game has only real people involved, while they are all made up and fictional in "24". And I know that the "good guys" will win and the "bad guys" lose in "24". I don't know that in the soccer game, the "good guys" being whichever team I like support.
And with the growth of computer games, their content has become as varied as the people who play them. Computer games are no longer a small side-business made exclusively for nerdy 12-year-old boys. The average gamer is now, what, 30 years of age or more? The time is past when you could profile an average gamer, because just about any sort of person plays games on his or her computer now. And they are all attracted to different kinds of games. And with option of playing online, the community of gamers will spawn interesting things by the mere virtue of their presence and numbers. In an MMORGP, the developer doesn't need to program the bad guy, because someone will want to BE the bad guy. And he or her will be powerful, because others will compete to take that title away. With millions of people playing, the community begins to take shape and become a world onto itself, with its own rules and laws and wars and alliances. You no longer need a governing plot to dictate how things will unfold then. This mirrors the real world more than old plot-driven games. And they have no plot. Because the real world doesn't have one either.
Are they better? You'll have to answer that for yourself, because it's different to everyone. I like plots. I can't bear to play Diablo II. And I have never played WoW, EVE online or Second Life. Not yet, at least. But I have spent hours upon hours playing through KotOR, TSL, The Summoner, and loads of other plot-driven CRPGs. Why? Because I found the plot compelling and needed to see what happened in the plot next. So for the me the answer might be no. But is the plot important? No, it isn't. More than eight million people playing WoW would seem to suggest quite clearly that it is not.