Metalanguage - should we use it in the classroom?

We waffle around the issue of whether teachers should, must, ought to or (add in a modal auxiliary of your choice) use metalanguage when teaching. How do students respond to the teacher talking about past participles? Can teachers handle students asking questions about subjunctives? Do we tell the students about bilabial plosives? We also waffle about guitars and cars because they make cool analogies.