driving below 2800 RPM = trouble?

[Steve Tinker]

Think of it as riding a bicycle.
Travelling @ 5mph in your lowest gear your legs will be revolving rapidly - not a lot of pressure on your leg muscles but you will run out of breath & tire quite quickly.
Now try pushing the same bike in top gear @ 5mph - you will probably give yourself a hernia & rupture your knees trying to turn the pedals - thats the same as lugging the engine. With low revs & WO throttle position the load on the lower engine componants (crankshaft, big end bearings & crankshaft main bearings) will be severe.
And don't compare a high compression, high revving sports car engine like a Porsche (who's max torque is @ about 4500rpm) with a diesel engine designed for low rev power who's torque curve is virtually flat from 2000rpm to 5000rpm. They are a different animal all together.