There are many reasons for not handling a tarantula, the most important in my opinion is to prevent injury. T's a surprisingly fragile especially when it comes to falls, just a fall of a few inches is enough to kill them. It doesn't take much to rupture their delicate abdomens and once that happens, it's pretty much game over unless you're fast enough with superglue (yes, you did read that right!).
Personally I have no opinion on if it should be done or not, I feel that's up to each tarantula owner to decide for themselves. Although it would be cool for me to pick up my pretty 5"+ T, I prefer keeping pretty and safe in her terrarium
Pixie