Interesting minor characters can give a book a well rounded feel.
Just like life, people can make the difference in terms of color, humor and diversity.
The minor characters should be interesting in proportion to their prominence in the book. If they’re made too interesting the reader will be disappointed when they don’t figure in the ending, when they don’t get more face time or when they don’t develop.
Think about it. If the character is that cool why wasn’t he or she featured more? It doesn’t make sense to give these characters fascinating backgrounds and then not write enough about them.
Whether you want to believe it or not, readers cry out for structure in fiction. Part of that structure is having things make sense. A minor character should be minorly interesting.
It’s why some minor characters can be cliched. A cliched minor character barely registers in the mind of the reader. The gruff cab driver, the nervous drug addict or the barely verbal Mafia muscle guy work because you don’t have to think about them.
Make them bigger and the cliche becomes annoying.