But you do this automatically when you are trying to discern the truth - invalid arguments are to be eliminated.
The whole, now argue for the other side is kind of ... frankly, telling the anyone to argue for any side is ridiculous IMHO. You are pretty much putting the cart before the horse - i.e. picking a position before the arguing has even started.
The whole point of discussion and debate should be to decide which position to pick (because it's the factually correct one).
The point of this exercise could be to learn that "nine out of ten dentists" arguments are flawed and why. It could be to practice listening skills to be able to "flow" the points of discussion.
These practices might not be to get at the truth of any particular argument, but to learn methods to get at the truth of any and all arguments.
If you are in a courtroom, say, serving on a jury, you will be presented with carefully crafted discussion and debate which will make you think the best and worst of both sides. The lawyers presenting those cases could switch at any moment and present to you the opposite side's case. Does the opposite side not deserve representation? How many false convictions resulted in the death penalty because a lawyer failed to argue their client's case well enough? How many pagans were burned at the stake, or Christians crucified and fed to lions, because people were taught that argumentation was to decide which position to pick? Discussion is to open your eyes, not close your mind.
The world would look a lot different if this were something people do "automatically". No, it is laborious and emotionally difficult, and even smart and intellectually honest people who try hard fail at it often.
> The whole point of discussion and debate should be to decide which position to pick (because it's the factually correct one).
Something in this formulation reminded me of those times at university - I was going through some sort of crisis during the first year. In my country at the time, elementary school and even high school approached things kind of like "there are things you don't know yet and once you learn the facts, you will know the truth". University destroyed this for me. I started to realize how flawed our brains are and how complex reality is. There are facts that are and always will be beyond our grasp and we can only have approximations of approximations, simplified models that hopefully work well enough to be useful. It was surprisingly emotionally difficult for me to accept that fundamental insecurity and un-discoverability of some ultimate "truth".