As a reader and a writer, sub plots nearly always add to the strength and expectation in the storyline. Sub plots can be used to further explore characters in a different context or add intrigue to the main plot. As a reader I love complexity and something that adds to the experience. Having said all that, a well written story does not necessarily need a subplot. It depends on the quality of the writing. As a reader I probably prefer the value of subplots as they can hook you in even more.