One of the most controversial changes that has happened in the history of League of Legends has to be surrounding Exhaust. It got smacked hard with the nerf hammer and many folks were not pleased with the changes. It no longer shreds 10 armor and 10 magic resist so people find it useless.
According to Wav3break, a Game Designer and part of the Live Gameplay team for League of Legends, he says that Exhaust was never meant to be that powerful. "Exhaust felt like it doubled down too hard as a damage reduction and damage amplification tool," said Wav3break. "It already flat out reduced damage dealt by its target and its slow is potent enough as a catch tool for focusing down enemies in many situations. In a sense, we were trimming some of the fat off of Exhaust that we felt made it too universally potent as a summoner spell choice — we wanted choosing Exhaust to be part of your strategy, not just a default option."
With that being said, Wav3break believes the game is in a better place since the change to exhaust. "This was not part of the plan to enhance support champions and we recognize that the change does lower the power of support picks who relied on its efficacy interim, but we feel the game is better holistically for more roles after this change."
WWG's League of Legends Writer and Analyst, James Bates, believes you should still carry around exhaust. "Exhaust removes a target damage-dealing champion from the game for 2.5 seconds. That's the long and short of it. If that doesn't sound like one of the best things you can be doing in League of Legends then by all means, start taking Ignite every game instead, or whatever other spell suits your fancy if you're a mid laner. When the fed enemy Talon, Zed, Ahri, etc. kill you by looking at you funny, though, remember that you chose the path you're on. Nothing makes a fight easier to win than making a 4v5, and that's exactly what exhaust can do. No other spell offers anything close to that save Flash or Teleport. If you like LP, keep taking Exhaust and keep punishing overeager assassins."