I suppose the answer to this depends on what you mean by theory.
Theory can mean academically serious, peer-reviewed thought about why something works the way it does.
Theory can mean "I just made this up, but it makes sense from a certain point of view."
Much of the theory in NLP (neurolinguistic programming) has roots in the first type of theory. But that type of theory changes as knowledge grows. Theory changes as information supports new ideas. Common beliefs about how the mind works were not common forty years ago. Current ideas about schizophrenia, for instance, are quite different than the theory put forward by Gregory Bateson so many years ago.
If you read "theory" about NLP now you'll find there are two camps. One camp is desperately trying to party like it's 1979. They want to stabilize what they helped to develop forty years ago but many of "them" want to do it by teaching the same theory as they used to support the practice then. It doesn't matter that practices like meta-programs, sensory preferences and eye patterns do not have a theoretical basis outside NLP. They remain useful constructs according to the people who teach them. Just don't scratch too deep or you will find that the theory supporting them is as thin as the supporting research.
On the other hand, the relationship between physiology, sensory coding and language is now supported by theories across many fields. It's not always the theory taught in NLP courses, but there are interesting conversations going on about how the mind/brain/body system functions, and NLPers could contribute to those conversations. They are contributing in organizations where the leaders seek cross-pollination with thinkers from related fields. I'm thinking of organizations that combine neuroscience with NLP, and of the research in therapy that overlaps various theories from mainstream mental health or positive psychology.
It's why I read mostly around the field of NLP (rather than in it). There is no forum in NLP itself where theory would matter (there are associations that would like it to matter, but they are usually promoting theories that go back to the early days of NLP). There is no peer-reviewed forum for original contributions to the field and there are few opportunities for thinkers to explore alternative explanations for the observations generated by NLP without immediately being decried by the purists who are quick to say that any way other than their way is not really NLP at all.
Most of the books are practical rather than theoretical. This doesn't matter very much to most of the people who come to NLP looking for practical, hands on training in communication and choice. Theory is not the only thing that makes a field worthwhile.
But. . . theory is a sign that a field is mature and resilient enough to discuss more than one way of understanding it.