Subplots, by definition, enrich a story. They provide prime opportunities for character development, and may keep the story from being tiresomely single-minded. (At the same time, they can put characters in what turns out to be the right place/interaction/mood/etc. for a new development in the main plot.) They may even end up contributing directly to the main plot.
Still, defining what is and is not a proper subplot can be tricky. The one big no-brainer - "a subplot is not the main plot" - aside, on what level do a set of related happenings become a subplot? Also, is a "subplot" the same as a "plotline"? Often, in casual conversation about a book, one hears reference to a minor character's "plotline," suggesting that a plotline is defined by the character to which it pertains (which, of course, would mean great overlap, as a scene may be important to several characters). One hears about "romantic subplots" and "family subplots." A subplot could also be seen as requiring a complete arc, just like the main plot.
To some extent, this is all semantics. You could decide to call any set of related events a "plotline" and only one with a distinct introduction, climax, and resolution a "subplot."
I was interested by Ms. Crook's suggestion that a subplot "should climax and end just before the main plot reaches a climax." It seems to me that a fair number do not. Some even end after the main plot does - the classic (if somewhat chauvinist) example being the hero who gets the girl after completing his big challenge, possibly even earning her attentions by completing said challenge. Comic relief-type subplots, too, may resolve at the very end of a story, often with witty last words. And what about books in sets? They often have subplots that arc through multiple books as well as those that resolve within each.
I've been resisting very hard using the Harry Potter books as examples here just because I do that a lot, but they really do show what I mean. We have the main plot - "Harry fights Voldemort" - and many, many smaller plots. Harry and Ginny get together. Ron and Hermione get together. Neville becomes awesome. One could see subplots (or plotlines, or what have you) with the rest of the Weasleys, Remus and Tonks (and the clear parallel between Teddy's sad little orphan story and Harry's), and even the Dursleys.
I don't suppose I have clear conclusions to draw here regarding subplots and what they are. I am for them, certainly. Ms. Crook seems to be a proponent of the outlined story - she suggests outlining subplots as well as the main plot, which obviously would mean deciding what qualifies. I tend to make one big, loose outline, and include events if I think they are important or am afraid I will forget them. Sometimes, clear subplots emerge; sometimes, connected events that seem like a subplot end up being part of the main plot. The holistic approach to story planning has worked well for me. For example, if part of a plotline must be changed, it often affects things outside that plotline; separate outlines could make it harder to realize all of the things that will need to change. What I will conclude, then, is that as long as they are done well, it's fine not to define subplots too strictly.