A trainer who is a subject matter expert naturally has instant credibility with participants. However, it is not necessary for the trainer to be a content expert.
In addition, just because someone is a subject matter expert, it does not mean that person is a good trainer. Having expertise and effectively transferring that expertise to another person does not happen automatically. Training design and learning facilitation are skills in their own right that require significant expertise.
A good training designer can work with a subject matter expert to create a training program full of credible content that is also structured to increase the probability of learning. In this collaborative design process, the subject matter expert provides the content and the training designer develops the learning structure and activities to ensure that the learners attain the desired level of learning in the content. The involvement of the subject matter expert in the design of the program provides the necessary credibility.
Yes, there certainly may be some highly technical and complex content that would benefit from the presence of a subject matter expert in the classroom- not necessarily to conduct the training, but instead to be available to answer questions. However, if a subject matter expert cannot be present, participants will be satisfied as long as the trainer is honest about what he or she doesn’t know, is willing to follow up with the expert, and gets the desired information back to the learners.
It is ultimately not important what the trainer knows. The key is to create a credible context in which the learners can discover the content (originally provided by a subject matter expert) and create meaning for themselves.