We got taught 8 & 4 to stop the airbags breaking your wrists upon deploying. That and it stops your hands getting pushed back into your face and breaking those pretty noses! Gives you a better chance of walking away post incident.

 

Resting your hand on the shifter will put wear on the shift forks inside the transmission, and occasionally you may pull it out of gear when the transmission unloads. Shifting with out the clutch is only detrimental if you don't match rpm properly, either grinding the gears or not being able to get it into gear (this is where you are wearing/damaging the synchros). If it slips into gear with no fuss there will be no more wear on the synchro than if you pushed the clutch in. The sychro's job is to slow down the gear using a little friction to allow it to mesh smoothly with the gear that was not turning (ie the gear you are shifting into).

 

Also, if you drive with your hand on the stick and wear all that stuff out, it'll drive you insane with all the rattling and squeaking it'll do later on down the road. Hop in an old big rig for a ride, see how you like how it sounds.