If you do, your main character will become less important than the supporting actress. Supporting characters must affect your main character but may never assume omnipotence. Remember, your reader is subliminally the main character and is not paying you for a degrading experience. Writing more than a few sentences to describe them and their exploits gives them too much space in the reader's mind and critically dilutes your main character's importance. Sub-characters are merely air people who float around the edges of your main story and provide the banks of the river of events which affect the main character.
Water flows through both.
The difference is their degree of cohesiveness. They are both flowing water but neither has the efficient flow of a large straight pipe. Your book should be like a pipe from beginning to end. Your book is a pipe. The Mississipi flowing to the sea has but one point.