As an experienced practitioner I would question the use of the term Conspiracy theory with regard to the issues surrounding NLP given that the evidence is so clear. Bandler and Gringer were supervised at Santa Cruz by Gregory Bateson who had previously worked on secondment to the US intelligence agency on project MKULTRA, the CIA mind control experiments. It is reasonably clear the much of the work in NLP in terms of changing perceptions and modes of thinking comes in large part from the research within MKULTRA and that the overall effect of NLP can be likened to a form of brain-washing. Given the lack of control over the number or background of those trained in NLP through the 1980's and '90's it will come as no surprise that there are organisations now gaining respectability with the mainstream psychology community whose ethical position with regard to the use of NLP is highly questionable.
There is far more to NLP than a harmless therapeutic technique. At its ultimate level it combines linguistic psychological control with manipulative body language and subliminal conditioning techniques to create a powerful and insidious control mechanism. The biggest issue however is that NLP is becoming widely accepted as a mainstream branch of genuine psychological techniques rather than the sinister cult that it actually represents.
The thing is, what holds NLP together is pretty much bullshit that was lent credibility by the established procedures and known phenomenons from psychology and psychotherapy. You don't need NLP.
If you want to truly improve your relationship with other people, be respectful, interested and try to see the world through their eyes as much as possible.
"How to win friends and influence people" by Dale Carnegie is worth it's weight in gold - it will help you to build actual relationships. NLP books are worth what you already have in people knowledge, and often transform their followers into lifeless, phony masks of people that have a weird handshake, grin and nod too much and are all around eerie.