The DirkGently-Killing Game-Theory-Reading Teamsters Union

[onering]

Deals can definitely benefit both players, and while it will generally benefit one player more that's not always a problem. Indeed, imagine a scenario where a player offers a deal in order to not die right then and there, like "if anyone can kill that creature that's attacking me for lethal, I'll draw them 5 cards with Stroke of Genius. The player offering the deal benefits most in one sense, simply because not losing is worth more than 5 cards, but the player trading a kill spell for 5 cards might as a result suddenly be in the best position to win the game. That might be a creature they needed to kill anyway eventually, and now they're getting 5 free cards in exchange for keeping a player alive that might still be pretty unlikely to win. Both players benefit, the saved player benefits more, but the lesser benefit enjoyed by the second player is still significant enough to make it worth accepting the deal.

Also, this is a game of imperfect information. The person offering the deal does not know everything, and neither does the person to whom the deal is offered. The person offering the deal will most likely believe that they will get the better of it, but they don't know what's in your hand or what your deck is supposed to do. Maybe you planned on doing what they are asking eventually, so you'll essentially be getting things for free just to speed up your timeline. Maybe what they are offering is worth more to your deck or game plan than they realize, for instance if Dirk offers you 5 hippos to do something for him but he doesn't realize he's setting you up to cast Craterhoof and win on the spot next turn because of it. Maybe someone is asking you to help them do something you were hoping they'd do, like asking you to remove a blocker so they can knock out another player, which will then allow you to win. Maybe the person offering the deal doesn't realize your holding a means to negate what they will get out of the deal. Because Magic is a game of incomplete information, player skill and experience matters a lot, as less skilled/experience players are more likely to offer bad deals as they are less likely to anticipate what others are trying to do and less likely identify possible ways their deals could go wrong and properly account for that. A player like Dirk, who is both skilled and very experienced, is rarely going to offer deals that can backfire on him, he's probably not going to offer 5 hippos to a green player sitting on 8+ mana with a deck that probably runs Craterhoof without some way to stop it.