I agree with Debbie. Anything that isn't in some way related to the main plot shouldn't be part of the book. You're making a bargain with your readers' expectations: 'whatever's in this book, is in here for a reason.' If your supporting character's side trip to the Great Goblin Market of Cornerstore for chips and beer doesn't mean anything to the plot, then it cheats the readers. And believe me, they'll pick up on it.
I'm a D&D player, and I had to realize early on that writing a story and crafting an adventure are two different animals. Depending on your party, side-adventures can be for the fun of it, the profit, to stall the players while you're figuring out the next major encounter, whatever. Stories have to be more self-contained. Dragonlance and Salvatore aside, D&D isn't always the best template for fiction.