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What are your views on conceding and rage-quitting? I've seen people quit games they could have and probably would have turned around...
Is it polite to quit and not make everyone lose their time? This is a game and losing can be a learning experience.
Not going to address rage quitting as, in the context of this particular game, conceding is something altogether different. Concession a practice that has it roots in paper Magic and other games (poker, chess) where players recognize they are in losing positions and act accordingly. Now if your claim about learning is limited to players who concede because they are in a difficult rather than losing position then there is merit to your position. However if your claim covers all concessions then you have a limited view of where and how learning takes place in the context of a Magic game. Maybe some individuals need to see their life total hit zero to learn but it is more likely that it is irrelevant to learning. A player recognizing s/he is already in a losing position and why they are their is more than a sufficient condition for learning. So if by losing you mean life total = 0 or being unable to draw due to not having any cards in your library you significantly over-estimate the value of losing.
Sports also demonstrate how conceding or at least not playing to the bitter end is part and parcel of most if not all games. Baseball: stars are traded when teams realize they cannot make the playoffs. Yeah yeah it is so they can rebuild for next year--and it just as likely that they are dumping payroll. Star players are pulled from games. Oh right, that is to prevent injury for the next game--yet you do not see them fighting to stay in the game. So much for playing until the bitter end. Players stop training and practicing as hard. But that is not the game. Sure it is, when less attention is devoted to practice, film study etc. it impacts performance. The idea that, in sports, teams and players play to the bitter end is belied by the facts that team management, coaches and players engage in any number of practices that indicate winning is not a priority. Simply because players do not walk off the field before the clock reads 0:00 does not mean they have not given up well before then.
When it comes to conceding it is not a question of politeness or impoliteness. In the context of games like Magic conceding and playing until the end honorable ways to end a match. Want to make me earn my win, no problem so long as you are not wasting time (unnecessarily pausing, letting clock run, etc.) no problem. Need your time to think through lines--take it. I think it is safe to say for the majority of the player base would agree with that. But beating someone who wants to stick it out is not more rewarding than accepting the concession of the player who recognizes they are in a losing position. As for those that miss an opportunity to learn because of quick concession, I simply move on to the next game. There are better things to worry about then trying to ensure someone else's learning experience in a game that has little consequence.