A multi-plate motorcycle clutch works on very much the same principle as a manual clutch in an automobile. "Slipping the clutch" does cause excessive wear. The clutch is not used for speed control. Proper gear selection and the throttle are.
Actually, you are both right. In normal riding, use the clutch as you would a car clutch, without slipping it. But the MSF teaches a slow riding technique that I call trickling. With the motor at an idle, or just above, if necessary, ease the clutch in to move the bike forward, pull the lever to stop. It is a great drill for beginners to give them a feel for the engagement point.
Ride all around the range, using the clutch to start and stop the bike. Most bikes in good tune will tolerate this. Some lack the necessary torque at idle, and need a very light application of throttle. The clutch slips a little, but it is not abusive.
The other case that calls for deliberately slipping the clutch is off-road. On a steep climb, you may lose enough speed that you are in too high a gear, while, for any number of reasons, you are unable to downshift. Fanning the clutch while applying throttle will get the engine up into a rev range where there is more torque, and the bike will climb where it otherwise would bog down. Yes the clutch will slip, and you may well consider friction discs to be a consumable item, even though you might put 200,000 miles on a street without replacing the clutch.
I assume the OP’s Suzuki 400 is the DRZ-S. It is actually a wonderful touring bike, for the right conditions, but I would not presume to tour one without a custom seat, or at least some modifications. Something like Sweet Cheeks, a cordura throwover device, uses plastic water bottles to support the inner thighs. It can add many, many miles to your touring day.
To tell the truth, very few stock seats, even on touring bikes, are up to the rigors of serious touring.