They are as much as a reward as you make it out to be....
OCD and achievements is a completely different matter. I would go so far to say that the way achievement systems are implemented is actually hostile to people with OCD and similar mental disorders.
"They are as much as a reward as you make it out to be." discounts the fact that the psychological conditioning designed into the system works on your subconscious mind. An Example:
Your first (ever) achievement probably doesn't mean much to you.
I beat the first level: Ding - Pop-up
Whatever right? But over time, you become conditioned:
I beat the second level: Ding-Pop-up
I beat the third level: Ding-Pop-up
I beat the fourth level: Ding-Pop-up
Every time you feel good about something you've done in the game, you now have the ding-pop-up associated with it. Piggy-backing on your own sense of accomplishment or relief you would naturally have for completing a stage in a video game.
Fifth level: Ding-Pop-up
Sixth Level: Ding-Pop-up
Final Level: Ding-Pop-up
Congratulations, you beat the game on medium. You loved it. You want to play again!
Beat the first level: Nothing
It didn't mean much to you the first time when you got the achievement for that level. But you no longer feel the same about beating the first level. With the association your mind has made with the Ding-pop-up, your subconscious is no longer validating beating the first level as an accomplishment.
So what do you do? Check the achievements list. Already, your behavior has been modified. You would have happily just replayed the single player. Now you are looking at a menu of options you have remaining for the ding-pop-up.
The coercion is a but more subtle than that. But it is coercion, manipulation and conditioning in design and implementation. They are designed to keep you engaged in the platform and ultimately, when the achievements run out, buy more games.