Hidden Serves
At the higher levels (i.e. 2600 and up), most players hide their serve because most umpires simply are not enforcing the rules. The main rule in question is, "It is the responsibility of the player to serve so that the umpire or the assistant umpire can be satisfied that he complies with the requirements of the Laws." Many players have learned to just barely hide contact from their opponent, but they do it so quickly and subtly that umpires, sitting off to the side, aren't sure if they have hidden the serve - and instead of warning and then faulting the player for not fulfilling the rule quoted here, they let it go. And so those who cheat are rewarded.
There are always exceptions, such as world #6 Vladimir Samsonov, who never hides his serve. How good would he be if he did so? But he plays against hidden serves regularly, and developed his game before hidden serves were illegal, and so can return them effectively.
Before, illegal hidden serves was mostly a problem at the highest levels. Now it's spreading to the cadet levels. It's survival of the fittest, and the "fittest" are those who win, and more and more these are the ones who hide their serves.
The problem is you cannot learn to return hidden serves unless you practice against them on a regular basis for a long time. (It's not easy learning to read spin from the way the ball travels through the air and bounces on the table, and to do so quickly enough to react properly.) And you can't do this unless your practice partners use them. So the only real way to teach players to return hidden serves is to teach them to all to hide their serves. Plus, even if you learn to return hidden serves, you have to use them yourself if you want to compete evenly.