"I was not referring to physical exhaustion. I was thinking more on
the fact that they don't have to go through years of training for
their job."
ahhh, mr pacman, that's where u are slightly off the mark. Coming
from a person who is quite intimate with the sex industry,i know
for a fact that the professionals prostitues, aka those that work
in the legalised whorehouse, recieve training prior to their
posting to overseas attachment. The rudimentey basics like hand
jobs and blow jobs just to name a few, are taught stringently by
their masters and employers. Most girls, to a certain extent, does
have trainings and woe betide the girl who know nuts about the
working of sex if she starts work in our legalised brothel. ha!
imagine a secretary who can't type, u catch my drift? 
And one more interesting thing to add, the act of making out
involves skill and girls who are skilled in this tends to command
higher premium. Of course, this has to be tempered with looks too.
I do know of one girl who happens to be under my charge not long
ago who looks like Cherie Chung and make out like a nymph who have
been caged up for years...needless to say, she's extremely popular
and "expensive"
------------------
The Far East Pilgrim