I find it a little silly that someone would complain that a harder difficulty makes the game longer. Surely, there must be a point where there must be at least some difficulty, or else it is no longer even a game at all. Imagine a game where, if one were to select the easiest difficulty, the game would just simply say "Congratulations! You win!" and shut off the console. I certainly wouldn't ever consider "playing" a game like that. Suppose, then, that someone is playing the game for the story. In this case, maybe the "easy" button would just skip through all of the "hard" gameplay, and just play all of the cutscenes in the game in order. You get your story, there's no chance that you're going to lose, and everything is fine, right? Though this might be somewhat enjoyable for a few games with truly fantastic stories, it wouldn't even touch the actual feeling and atmosphere that those same moments have after having played the role of the character - particularly any challenging aspects of being such a character in such a situation. The challenge of a game is what separates it from just being an interactive movie.
Now, on the other hand, I can understand choosing to play on normal difficulty rather than hard or impossible or insane or whatever the case may be difficulty. It provides enough of a challenge that the game is still a game, but you don't need to spend hours practicing or number-crunching mechanics in order to move forward. Now, I can understand, if one is looking for a challenge, taking the harder route - having to do that extra strategizing or having to spend those extra minutes of practice are simply taking the challenge of the game and moving it forward to a new level. Of course, if someone has trouble with these harder difficulties and finds them frustrating rather than fun, by all means they should adjust the difficulty to the point where it reaches the level of challenge that is steep enough to make one need to use their skills at the game at an acceptable level whilst still having fun.
I, for one, never cheat on a game unless I have worn out every other aspect of it and have additional things to gain by doing so (for example, accessing areas or content that normally is not available except through hacking, glitching, cheating, etc.) It actually makes me sad to see people who cheat through games, particularly games that I really enjoy. I can't even put my finger on exactly the reason; maybe I just take my games too seriously. But it's awful, after playing through a classic like Zelda:OoT or a more modern masterpiece like Mother 3 or Persona 4, seeing someone who is cheapening their own experience of such a work. Sure, they are getting all of the same story, same characters, and so on, but they're missing out on the real gameplay! Even just small cheats like unlimited money in an RPG in order to get rid of grinding remove so much from the experience - in this case, balancing limited funds to decide where upgrades are most needed, and perhaps even having to do without some of the more expensive items unless one has the time to grind out the extra gold. Granted, playing on easy mode is usually not quite the same as outright cheating, but in many cases, it isn't far removed (one example that comes to mind is Megaman Legends, where easy mode starts the player with the strongest weapon in the game, which is only obtainable on such a mode. It's so powerful that there's never a reason to use a single other people of equipment throughout the whole rest of the game, because you can beat anything by holding down the shoot button for five seconds or less.)
On a less-ranty and more introspective note, perhaps it all comes down to why people choose to play games. The end result, with probably very few exceptions, is to have fun. Some people might find that fun in blasting through their enemies by mashing a single button without having to spare a thought for strategy, whereas others might find the fun in the planning and careful execution itself. People are just inherently different - one person might enjoy spending time to build a snowman, and another might have more fun simply pushing the snowman over. In conclusion, (does this sound like an essay yet?) it seems to me that the people who play on easy mode are either 1. physically or mentally lacking the capability of playing the game as the developer intended, and thus in need of a dumbed-down version to be able to get any sort of enjoyment from the game at all, or 2. finding their joy not from the game itself, but from the baser human desire to "win." They love the flavor of winning, even if that winning was in the same vein as a preschool game in which everyone is the "winner" so as to not upset any of the four-year old children. Those who play games on a higher difficulty - at least normal - are hoping to enjoy the game in the way that the developer intended and having fun by actually playing the game, whether that be through mastering reflexes, coordination, or game mechanics and strategy. One might argue that they play on easy because they don't enjoy the game enough that they want to devote time to actually learn to play it, to which my response would be to simply put the game away - return it to the store if you must - and go find something that you will enjoy enough to actually play.