A comic without a sub-plot is boring as hell. Most of the action and excitement should come from the main plot, that’s a given, but without side plots to show us the characters and learn about the setting and back stories, the whole thing turns a little stale.
The most interesting thing about sub-plots is that they let the team behind the comic cut a little loose. As it’s the ‘less important’ part of the story, a little more artistic freedom is generally allowed, allowing for – dare it be said – fun.
Sometimes, it’s maybe a little bit too much fun, and a major superhero is given an unexpected drinking problem, or Death gets herself a harem.
Although these side plots can get absolutely crazy, they’re almost always worth it as they’re done entirely to be interesting, instead of to carry the plot along. The line between interesting and just plain bizarre is blurred fairly often, sure, but when the exchange is getting to read about an assassin whose arsenal draws solely from dessert pastry, it’s more than a fair deal.