Think of it more as a gamer history. Plus, 'unfinished achievements' say a lot more about the game and the value of it than deleting it does.
Rod Fergusson, former and now current producer of the Gears of War franchise, talked about how they (Epic) would track the achievements of players, and used it to let them know about what, how and if people were playing the game. He explained that most other studios do the same. It's why Gears of War, and especially the sequels, have so many simple achievements for playing through the campaign. It lets them know how far people play through it, where they may stop, and potentially why. Maybe the level is too hard, or the player just got bored of the game.
This is why Gears of War 2 had an easier difficulty selection and why cooperative players could play on different difficulties, as they could track the online achievements of players from the first game and see the ratio of people who completed the whole campaign, who finished all of the difficulties, who played through with another player and, against that, those that did not.
So don't worry over the insignificant number that just shows how many games you have played, and what you might have done during those play times.
