Such suspicions are well-founded, and very often correct. Mocking achievements is good. And yet, there also good achievements, achievements that do add something to the game experience. So let us look at what is good and what is bad. (Recommended reading: achievement design 101 at Gamasutra.)
Many games ask you to perform at a certain level before you are allowed to continue. In a shooter, you must get through the level without losing all your health. In an RTS, you must defeat your opponent. The game recognises this by showing you a "Victory! Loading next level"-type of screen.
But it is of course possible to adopt higher standards of performance, in order to make the game more challenging and more fun for better players. Achievements are a good way to implement this: not earning an achievement still allows you to continue the game, so less good players are not penalised; but better players are rewarded for taking on the additional challenge.
Starcraft 2 is excellent at this. Every single-player mission comes with several achievements that make it tougher. For instance, you might be playing a defensive mission, where you have to survive for 20 minutes. One achievement is awarded if you don't lose any buildings during that time: this forces you to get a better, more organised defence. A second achievement is awarded if you destroy four of your opponent's hatcheries (big buildings). This forces you to somehow get an attack force together and do counterattacks while still defending your own base. Difficult, but very satisfying when you pull it off.
Teaching the game
By setting a goal, and asking players to attain that goal, you can teach them an essential gaming skill. For instance, Starcraft 2 has an achievement that has you
Train 10 marines during the first 320 seconds of a single Melee game.The only way to train 10 marines in 320 seconds is to really optimise your build. You have to think about and experiment with different tactics: for instance, should you spend you early money on more marine-making buildings, or is it more efficient to spend it on marines immediately? When you have earned this achievement, you have not just learned a useful in-game skill (getting an army very quickly), but you have been introduced to an entire way of thinking about your early-game strategy.
A certain amount of feedback on and recognition of your progress in a game is often desirable, and achievements can be used to provide this if the basic structure of the game does not. That latter part is essential: if the game obviously recognises your progress, achievements that do the same thing are cheap and useless. The Valve-games (Portal, Half-life 2) are culprits in this regard: the whole structure of the game is that of linear advancement, so you get continual feedback and recognition; and yet Valve has added achievements of the form "you have reached part X of the story!". Useless.
Much better are the song-by-song ratings in Guitar Hero-type games. If you get better at the game, you will manage to beat your previous records. This gives a sense of gradual advancement that would not otherwise be present in the game.
Superfluous progress recognition
See above. An achievement that you get just for advancing in a game, where advancing in the game is already an obvious reward for your success, is a useless achievement that feels cheap. It makes the player wonder whether the designer really thinks the player is going through the game just to earn achievements.
Achievements for things that you cannot really set out to accomplish are bad. Team Fortress 2 has quite a few of these. (In general, I am underwhelmed by the Team Fortress 2 achievements.) Consider the "Rasputin" achievement: In a single life, get shot, burned, bludgeoned, and receive explosive damage. There is no sensible way to try and get this achievement; it is just something that will happen to you if you play the game often enough. Or you can join a special achievement server, where people help each other getting these random achievements...
Random achievements do not reward skill, and if you do get them, they do not feel like an achievement. The only thing they reward is perseverance, which is exactly what makes us suspicious about achievements. Of course, just because you cannot set out to achieve them, they are still better than...
The very worst achievements are the ones that require no skill, but only a willingness to keep doing the same thing over and over again. Team Fortress 2: "Kill 100 enemies while both you and your victim are underwater." Starcraft 2: "Win 1000 1v1 Quick Match games of each race (as well as 1000 random games)". World of Warcraft: "Get 100,000 honorable kills". And special hatred goes to Half-life 2: Episode 2: "Squish every antlion grub in Episode Two.", an achievement that manages to combine tedious grinding with an miss-one-and-start-over system.
The only purpose of a grinding achievement is to make you play the game longer. They are not challenging, They are not fun. They are just a meaningless reward given to those who become addicted. And yet -- they are psychologically powerful. I have succumbed to some of them.
And achievements like "you have recommended this game to a friend on Steam"? Let's not talk about them.
A good achievement is an achievement that enhances the game. The game can be enhanced by adding interesting challenges, a bit like adding an extra level of difficulty; by teaching the player useful in-game skills; and by allowing the player to track the improvement of his skills.
Skill is the central word here. Those parts of the game that are not about skill (theme and story, beautiful art, interacting with other people) do not need achievements: it would be ridiculous to have the game say "Great! You have seen the crucial scene about how sex without emotional vulnerability may be fun but is not, in the end, really fulfilling!", or to have it say "Wow! You just had a good time with your online friends!". Games that do not rely on skill do not need achievements.
Bad achievements are primarily those that attempt to disguise non-skill as skill: "Wow! You have managed to kill 1000 rats!". The skill is killing a rat, but not killing 1000 rats. These achievements are cynically used to seduce players to spend more time with the game than they otherwise would. In a further act of cynical manipulation they are often combined with a system that allows you to proudly display your achievements to other players.
It makes one shudder to think of Diablo 3, does it not?