Gardner's theory has been enormously influential in schools and has attracted little support from psychometric research. Why is a theory's classroom popularity not evidence for it?

B. Because a framework can be useful to teach with and still be false.

The framework encourages teachers to look for strengths a narrow measure would miss, and those are good instincts whether or not there are eight independent intelligences. The psychometric evidence bears on the structural claim, and there the record is thin. Adopting a theory because its consequences are appealing is not inference.