Why you think it is beneficial for the server: Extra achievements give players more fun silly things to do and more goals to have when they get bored of typical Minecraft gameplay. It's an overall good thing for the server.
100% agree on this one. It would also be cool if we had a plugin that could check people's "stats" on their achievements (or even things like mob kills, blocks mined, etc.) and it would be a fun element to the game.
The first link I put above is probably the best one for that sort of thing, Obliv.
ETA to be implemented: 22nd June