In a 2-player game a resignation is just the same as a loss, ratings wise.
For 3+ player games a resignation is worse (ratings wise, by a factor of 1.5) than coming last in the normal course of the game. That's because the 3+ player game cannot be finished (you can't simply remove a player, and substituting a bot isn't good because the bot is playing badly/differently), and so the other players can't find out who would have won.
So even if you know you will be last, this encourages you to play on, so the other 2 players (in case of a 3 player game) can determine who wins and give the winner the appropriate ratings increase. You're bound to profit from this when you are the one in the lead and one person is hopelessly behind, and then they don't just resign.
It's the same with any multiplayer game: If one person leaves, the game is often crippled in that it can't be continued.