Theory is a big part of what you study in "therapy school" (whether you're a psychologist, clinical social worker, professional counselor, marriage & family therapist, etc.) and there are over 400 approaches to psychotherapy the last time someone bothered to count (which, I think, was back in the 80's, so I'm sure that number has grown considerably). Like others have said, most therapists incorporate elements from many theories. The important thing is that the combination makes sense and doesn't contradict itself. It ought to be the product of some reflection on the part of the therapist, and not just a "grab bag" of techniques and ideas that the therapist pulls from...or worse yet, just a fad that the therapist is enthralled with.

Many studies show that some approaches to therapy work better with some problems. (For example, dozens of studies support the idea that Cognitive-Behavioral Therapy and Interpersonal Therapy are the most effective approaches to the treatment of Major Depression.) But by and large, the research seems to indicate that the theory itself doesn't matter too much as long as the therapist has one and is using it consistently. Also, all the theoretical aspects are less important than other factors like the relationship, the client's perception of the therapist, the client's expectations for change, and what happens in the client's life outside of the therapy sessions. (One analysis suggests that theory-specific techniques only account for about 15% of the improvement people experience in therapy.)