Lugging the engine, as well as putting more mechanical strain on, also causes pinging, or detonation (pre-ignition), and does it when the oil pressure is at its lowest due to lack of revs - the worst possible combination.
If you look at a typical torque curve, you'll see on the left side the torque is relatively flat until revs increase and torque starts to improve. This flat area is where lugging occurs, as the engine is not getting enough fuel/air to make the power being demanded of it to accelerate the weight of the machine and rider.
It is not harmful to be in this area as such - for instance when decelerating, cruising downhill, or stopped at idle - it's wide throttle openings in the lugging zone that put demand on the engine to generate power which it is not getting fuel/air to be able to do.
Generally, it's best to be firmly on the rising torque area before applying heavy acceleration loads. You will likely get away with light acceleration on the flat or downhill, but it's bad practice anyway.
Personally, my bike is supposed to peak torque at around 3,000 rpm. I like to keep it in the 2,000 area to accelerate moderately, a bit more if I want to accelerate hard.
”This above all: to thine ownself be true,
And it must follow, as the night the day,
Thou canst not then be false to any man.”
"Generally speaking, you don't die on motorcycles.
You die a few yards away from what's left of them."
It appears I am not only a NRA Life Member, but also a Patriot Guard.
**** - I didn't know that
Holy Mololey, Batman, I didn't know I was a GOA Life Member, either
Being drunk seems to be better