I expect that the growing use, popularity, and acceptance of achievements in games is related to growing awareness of the Bartle Test of Gamer Psychology. Although still controversial, the test has been taken by enough people now (est. 700k) that estimations can be made about player populations.
In short, the test suggests that gamers, especially mmo players, fall into four distinct categories, with most players enjoying all four aspects but with one being strongly dominant. The four types of gamer psychology in Bartle are Achievers (players who collect points, equipment, etc. in order to measure success), Explorers (players who explore maps or lore for hidden information), Socialisers (players who want to interact with others), and Killers (players who want to compete directly with other players). The scoring creates a four letter code, similar to Myers-Briggs personality type, such as EASK.
There a really good, if slightly outdated, stats breakdown at
Overall MUD stats . What you can see there, as well as some other pages, is that Achievement is not often the primary attribute, but it is also rare for it to be the last one. In other words, most players will gain enjoyment from following an achievement based path, even if it isn't the primary reason that they play the game.
So I expect that from a game design point of view, adding "achievements" is a low cost method of creating appeal for most gamers, compared to adding other types of content. Achievements don't replace the exploring, socialising, and killing, but they complement all three. Likewise, out of game achievements, as opposed to actual gold, equipment, etc., provide any easy way for players to track their "progress", and act as an out of game reminder of their in-game enjoyment.
