Good question. In fact, this is a question that everyone should ask and be able to answer when choosing the different technologies and social, f2f contacts that ultimately become one's PLN. In my blog post I say, " If I choose and determine that an LMS is the best way for me to learn, then the LMS is my PLN". My point was that if a person can justify why an LMS best serves the individual's learning needs, interests, and learning preferences, then who am I to judge. I also say this because many people are for or against an LMS; I think this is the wrong conversation to be having. We should be addressing the question you pose that forwards this notion of articulating a learner's rationale in developing a PLN. A learner's rationale for using any tool will also depend greatly on how the tool is being used.
Clearly for me, an LMS is never my PLN. For this MOOC (and for the first time), I am completely staying away from Moodle and it's made all the difference! I weave in-and-out of blogs, tweet, and read The Daily in order to interact with individuals and content for the course. This part of my PLN works for me and I can explain why it works for me...but I cannot judge others if they can explain an alternative way that works best for them, including using only an LMS.
If someone says the only place they learn anything worthwhile is through some Moodle course, online community, etc., who am I to argue. Technologies today are so integrated anyway that everything really is just varying degrees of a PLN. Think of all the different ways online content can be brought into Moodle for example.