Know your opponent, know yourself and you got the key to victory, I never expected a quote from ash to be so true.
I must agree that the level of secrecy exposed or suggested here would be going to far, being mean to other players is not a wise choice, since the game objective is to have fun, personal grudges should be left for the ring, but random unknown players don`t deserve such treatment.
But getting into a logical perspective, information is a double edged sword, a good player would figure out the opponent deck right after the setup, without third party help, during a tournament you can freely shout about your deck, with no regret, since nobody will be able to tech or choose a typing that beats you in the middle of a tourney... But before the tournament starts it would be the most stupid thing ever, and yeah many people actually measure the competition by trading and asking wich deck you play, but most of the time they just want to squeeze you with overprized cards they know you need.
I find it really dumb to change your deck right before a big tournament, 1 card changes the flow of everything, and most of the time the people who do this, does it because they got a "last minute tip" about a card that worked wonders, this information might be real for certain decks, but not for all of them.
But is far more idiotic to try your deck against good players, this is a game that involves many factors, and observation might be one of the most trascendent ones, like in pocker; the more you play against a good player, you are not getting closer to beat them during the tourney, you are giving them valuable clues to prevent your winning pattern, since they will memorize anything that proved to be a dangerous match up, and obviously a chance to tech something that might obliterate the counter.
New or bad players on the other hand, benefit greatly from playing against the big hits, since they have a chance to learn tactics and see decks perform at their fullest, giving them a baseline on how to build their decks or how to use certain cards... the sad thing is that most of the time this marks the beggining of netdecking, since the top player might be a netdecker only worried for winning, this might corrupt sprouting players into becoming like them, is healthier for the game to let them create their own versions, where they must figure out by themselves what works and what doesn`t, rather than letting them get the info from internet or cloning your decklist.