There are a lot of MCs who have little impact on the plot though, Indiana Jones being a famous one, for all his efforts he achieves diddly squat, it's fun watching him though.
Perhaps I didn't phrase it right, but I was really referring to "side quests", not the majority of the book turning out to be irrelevant to the main plot.
Do you know any examples of this? I can think of some novels where the MC is sent on some quest but then it turns out it was all a sham and the story is really about something else, but that sounds like pretty standard plotting to me... But I get the feeling that's not quite what you mean?
That reminds me of A Perfect Storm. Trawler skipper goes out into the worst storm recorded, goes through a dozen incredible episodes, and then dies. I use incredible advisedly, because I only carried on watching because I was thinking "well, it's based on a true story, so it must have happened". And then you get to the end of the film, and find that the only thing that's true is that the ship set sail, bad storm, never comes back. It lost its radio early on, so ALL the episodes in the film were made up. Ripped off!
My feeling is as long as you have a clear defined "why" then it should be easy to keep the reader interested. Your "why" is to help strengthen the relationship between two characters. That said, if there is a better way to accomplish the "why" while still staying within the main plot that tends to be the better option. I know in the story "stand by me" king has the main character go off on a tangent about his first short story he published (the character, not king) and then actually includes it in the middle of the text. I actually felt that one was done poorly and took the reader out of the story that was taking place.
You read a lot a Dan Brown dont you? This is like his bread and butter. (FYI i like him, so this wasn't meant to come across as hostile)
I often dislike (loathe, really) sidequests where they're used the most: video games. But side quests can be fun. The issue with side quests is usually what Link The Writer mentioned, that some divine Pause Button is pressed when the character says "excuse me while I kill those spiders in the cellar ", and when the world is unpaused no one ever noticed any heroic acts of cellar cleansing. In a video game that's annoying. In a book that would probably confuse the reader. By the time you bring the reader back to the plot he'll either have forgotten the plot existed or that he was already thinking the side quest was the plot. I'm all for side quests if you manage not to hit that pause button and have it mean something in that world. Maybe the side quest has something to do with the main plot, or at least affects it indirectly. Or maybe the character is "multitasking", solving the side quest while keeping track of the main plot and doing something about it here and there in the meanwhile. Maybe that side quest can even be remembered or referenced later in the story, with some degree of relevance to the plot or to a character. I think that can actually be quite fun, both to write and to read.
I like side quest type things (as long as they seem like a natural course of action). They give a nice break from heavy plots, and the change of pace can let you see characters and character relationships better/differently. Of course, it's good to have meaning behind things, even if it's only a character becoming introspective, or something of the like.
I'm just going to repeat what many have said. If you do it well it's good if you do it badly it's bad. Also remember not to make up one for the sake of it.
I think if the side quest ends up having nothing whatsoever to do with the plot, then yeah, it's irritating to me as a reader. If it's a page turner, I'll stick with the book, but even then I'll be wondering WTH was that all about??
Just make sure: 1. it is brief, yet exciting 2. it has a purpose either towards characterisation or plot 3. if it could somehow tie into the overarching plot -even better go for it