This picture is from the ever-useful Standish report (aka Chaos report.)
It indicates how many deliverred features were used - ranging from always to never.
How many of the requirements in your specification/backlog are going to contribute to the orange or red sectors? And how do useless (?) requirements get into systems when the same report tells us that many important features are never implemented?
A question for the experienced guys and gals reading this site - do you have some tips on how to manage this problem?