Just puttin' along
Your friend may have something there. Modern sportbike engines aren't really designed for cruising around town in a high gear and low RPM. Lugging it below the power band can cause some internal damage to the engine and transmission, not to mention premature wear on the internals. The "cleanliness" issue you mentioned has more to do with the oil gumming up the innards, the exhaust track getting carbonned up, and the intake getting fouled.
The instructor at the very first track day I participated in made a big thing about keeping the engine way up in the power band. Say between 8,000 RPM and redline for performance riding. Even higher for racing. It's better for the bike's engine, and the acceleration and engine braking is much better as well.
Now, I realize that humming along at 10,000 RPM/50MPH in first gear heading down to the 7-11 might sound a little outrageous, but in the long run it's better for the bike, the riderhas more control and let's face it.......it sounds good, too!
Anybody else want to wade in on this one?