Marcus observes that teaching demands that we set aside jargon and get back to intellectual basics. This forces us to grasp, in a non-technical and intuitive way, what's appealing and unappealing about a philosophical position or claim:
when teaching Kant's moral/practical philosophy, it's really easy to get sidetracked by Kant's technical terminology, etc. But, when teaching a first-year undergraduate course, getting mired in that stuff is a recipe for disaster. Students tend to tune out. In order to get them to tune in, the challenge is to explain, in the simplest and most intuitive way possible, what Kant is up to, and how his theory is philosophically motivated. -This 'cutting to the chase' makes us better researchers, Marcus notes.
I'm intrigued by a second point Marcus makes in defense of undergrad teaching: Undergrads, not having been heavily immersed in the discipline, don't take a problem or argument seriously simply because those in the discipline do. They have, Marcus points out, good BS detectors, and have to be won over to thinking that a position or argument merits esteem. Undergrads thus serve to keep their instructors 'grounded,' we might say, not taken to flights of intellectual fancy. That said, I'm ambivalent about this point: Sometimes this BS detector is also an immaturity meter: Students may not take an argument seriously due to ignorance (philosophical or otherwise) or philosophical inexperience. Indeed, part of our job is to help them see the force of unfamiliar or obscure points.
Why else is teaching more than a "necessary evil"? I'd add two points here: First, teaching can be a source of challenges no less compelling than those we face as philosophical researchers. As I've argued before, our profession would benefit enormously if our teaching culture were more like our research culture in certain crucial ways.
Second, there's been lots of talk about 'public philosophy', the public face of the humanities, and so on, in recent years. I'm often struck by how those who advocate for a more public role for philosophy overlook their most public role of all — their role as philosophy educators. After all, in teaching, we have a semi-captive audience with at least some willingness to be persuaded of the value of philosophy. Our students are our most important 'public,' and each time we teach, we are doing public relations for our discipline and our profession. Teaching is thus a necessary evil only if showing that philosophy is worthy of study and worthy of respect is a necessary evil too.