With time and evolution of one's life, other aspects become prime - such as work, friends, outdoor activities, etc. And when that happens, you won't be compelled to waste hours in front of a screen, virtually attempting to live. To conclude, due to aforementioned aspects, trainers help speed up the process of finishing a game and are MOST welcome for those who love the story behind a game, rather than the - at some point, repetitive - gameplay.
I agree with SunBeam! I also use trainers, and still enjoy the game.