Every car I've ever had was a stick. If that is true, id be in bad shape. I dnt think pullin on it or doing more than resting on it is a good idea but I have a ford racing shifter in my Shelby, that's where my hand is if I'm in the car.
I really think it's an old-school issue that people still believe.
Way back when, cars didn't have shift-stops. Out modern trannies to so that you don't overshift and jam the gears together and I'm betting that they help with pressure on the shifter.
Like ubnbit said, as long as you are just resting on it, you should be okay.
I still don't do it just out of habit from driving my old Muncie Rock Crusher tranny.
The reason you should not do this is because the weight of your hand, and arm, is multiplied due to leverage. This can move the shifter rail slightly off the detent ball which holds the shift and fork in its proper position. The shifter fork fingers are then being rubbed against the rotating synchronizer sleeve (think of placing your thumb and forefinger inside the grooves of a small pulley).
Over time the shift fork fingers and the sleeve will wear down. Once the gap is excessive the force applied to the gearset in question (1/2, 3/4, etc) will try to slide the synchronizer inserts inside the sleeve. There is a point of no-return here and the synchronizer hub/sleeve assembly will then try to place itself in the neutral position; a.k.a jumping out of gear.
My grandfather taught me to not do that many years ago because it does apply an unnecessary load to the shift forks and synchronisation rings as maldog said. This could be a lesser problem with modern transmissions, however after 55 years of not resting my hand on the shift lever I have no plans to change now...