[Optional note that you don't have to read: the organization of this post might seem a little bit jumbled. I'm thinking in two or three dimensions here, but can write in only one dimension. And, y'know, it's a frickin' blog, so I'm not going to spend three hours trying to make sure the writing flows together.]
"Rosencrantz and Guildenstern Are Dead" is perhaps the most famous work that draws attention to what T-Rex is talking about here. The work follows Rosencrantz and Guildenstern as they play out their roles in Hamlet, having witty dialogues with each other when they are not on stage. They learn of their impending demise while on the boat to England, but learn too late to do anything about it. (I recommend that you read the play, and/or see the movie. If you see someone in your city preforming it, though, proceed with caution, because it requires some strong actors in order not to be dreadfully boring.)
T-Rex takes this idea one or two steps further: focus on background characters, and then have them interact with main characters. This is something you see very rarely, and it's only ever presented as something comedic. One example would be the "Scooby Doo" ending of Wayne's World, in which they pull the mask off of "old man Withers," who by that point you probably even forgot was in the movie. So why doesn't anyone use this device in a serious manner?
Well, as anyone who has even looked at fanfiction can tell you, fans are fucking crazy. Each of them is going to have his or her own idea of which main characters should date/boink/marry. Having a main character marry someone the audience has never heard of would be a slap in the face to most fans. If you doubt me, you should pay attention to the press's reactions to whom celebrities date. If a celebrity is already married when they become famous, no attention is paid to their spouse. But when they are already famous, it is inexcusable for them to date anyone but someone who is also famous. (It is okay if the fans haven't heard of the person so long as he or she is famous elsewhere, or just fabulously wealthy.) You're not allowed to fall in love with your makeup artist, unless your makeup artist also happens to be smoking hot, in which case he or she is probably going to start doing some modeling work.
This is why Brad Pitt and Angelina Jolie are Hollywood's darling couple: they are celebrities who are married, and it is working out so far! So many other Hollywood marriages crash and burn, but they are doing great! Of course, before Brangelina, the darling couple was Brad Pitt and Jennifer Aniston. So, y'know, let's hope things work out for Brad and Angelina.*
Now, let's suppose that you are making an innovative work of fiction that is defying tropes left and right, and, among other things, the characters have romantic entanglements only with background characters and supporting characters. You might develop a niche following, but most people will not be all that into it. This is because the things with mass appeal tend to use the same tropes simply because, if you want mass appeal, they work.
I am reminded here of when Adlai Stevenson was running for President. Allegedly, a supporter said to him, "You're sure to get the vote of every thinking man in America," to which he replied, "Thank you, but I need a majority to win." The analogy is thus: in the realm of movies (like "Star Wars"), a cult following is not what any major studio wants, because of the cost of producing the movie, and because so much of what gets you more work in Hollywood is being able to point to recent successes. "The Princess Bride" is a good example here: Cary Elwes went on to do "Robin Hood: Men in Tights," but nobody else in that movie did anything significant.**
However, for music and fiction writing, a small but loyal fan base is very good for you in the long run, because it gets you a small but steady income even after you stop producing anything. "Gravity's Rainbow" comes to mind, which is heralded as being one of the greatest novels of the 20th century, but the number of copies in print is probably not even close to a million. Elvis Costello broke into the US top 40 only once, but you probably couldn't name what song it was. (Hint: it was not "Pump It Up.")
Never forget that quality is a long-term investment.
* - SPOILER ALERT: they both die! But that isn't for like another 40 years.
** - Yes, Mandy Patinkin has gotten a steady stream of work since then, but if you can name something he's been in, then you are a nerd. This is not to denigrate you but merely to point out that because few people in America could name anything he's been in, that means he hasn't really gotten any major work since "Princess Bride." QED