Example 1. Alpha Strike from no Where.
When playing a MP game with an Attack Left or Attack Adjacent option, a situation might come up in a 4+ player game, with two opponents opposite of each other each with large armies (Lets call them C and A) of creatures and their adjacent opponents with no creatures (B and D). Player D (you) with a large army decides to do an all out attack because the person to their right is a buffer against them being attacked before their next untap step by Player A. Suppose that after you declare all your attackers, the player to your right decides to concede the game before the other person's turn with the large army. This might be because they don't have any answers in their hand to an all out assault from A's army. Now, because Player B conceded you are completely open to an Alpha Strike from Player A. This wasn't a tactically bad decision, it only became a bad decision because B conceded at a bad time.
801.2c The particular players within each player’s range of influence are determined as each turn begins.
801.3. Creatures can attack only opponents within their controller’s range of influence or planeswalkers controlled by those opponents. If no opponents are within a player’s range of influence, creatures that player controls can’t attack.
Example 2. Lifelink? You didn't do any damage.
In this example, you control a Baneslayer Angel enchanted with Armadillo Cloak. You are playing a 5 player game, and you have to opportunity to eliminate a player. You swing for 7 damage in the air, confident that with the 14 life (lets just say everyone is tapped out and you also have Shelter or Dawn Charm in hand) that you are going to gain from the attack that you can survive the coming onslaught from other players. After you declare Baneslayer as an attacker, the defending player decides to concede. This example could just have easily been Thieving Magpie (preventing a draw), Lightning Helix (preventing +3 life), etc.
800.4d If combat damage would be dealt to a player who has left the game, that damage simply isn’t dealt.
Was it a tactically bad decision to attack a player you could kill?
Should you be allowed to gain 14 life as if a goldfish was in place of the defending player?
Is ever ok to concede? Should a concession at anytime be treated as if it happened at the start of that players turn?
Example 3. I'm not going to win, so why should I make it easy for you?
Now, there is nothing wrong as far as I am concerned at striking a blow before you are inevitably removed from the game. I see nothing wrong with emptying your hand of burn in response to someone's killing blow, popping a Seal of Doom (or other Seal) on your way out of the game, but what about abusing cards in ways that were never intended? The rules even mention a good example...
800.4c If an object that would be owned by a player who has left the game would be created in any zone, it isn’t created. If a triggered ability that would be controlled by a player who has left the game would be put onto the stack, it isn’t put on the stack.
Example: Astral Slide is an enchantment that reads, “Whenever a player cycles a card,
you may exile target creature. If you do, return that creature to the battlefield under its
owner’s control at the beginning of the next end step.” During Alex’s turn, Bianca uses
Astral Slide’s ability to exile Alex’s Hypnotic Specter. Before the end of that turn,
Bianca leaves the game. At the beginning of the end step, the delayed triggered ability
generated by Astral Slide that would return Hypnotic Specter to play triggers, but it isn’t
put on the stack. Hypnotic Specter never returns to the battlefield.
So if someone threatens me with burn and I can't life gain my way out of it, I can just cycle a bunch of cards to remove my opponent's creatures, wait for those triggers to resolve, and then concede (or allow them to kill me)...bye, bye creatures.
Nothing prevents your opponents from conceding the game, and it can have large tactical decisions in MP. If your opponents don't think their is anything wrong with conceding a game (in fact the specifically tell you they are conceding to hurt you tactically), what do you think is the best way to handle it? I realize their are several solutions, but I am interested in what you or your group would do.