You Aren’t Going To Need It (YAGNI)
This is from extreme programming – don’t add functionality until you need it. Resist the urge to add things that you might need in the future. Only implement things when you actually need them, never when you are only foreseeing the need for them.
Usually if you start adding things you don’t need at this point in time it is a sign that your focus has drifted from the actual problem you are trying to solve.
Remember that your time is a precious resource, to add the unnecessary feature you are going to need to take time away from the functionality that you need to be implementing.
This directly impacts the quality of your solution, increases code bloat, makes refactoring harder, and can in some cases restrict your ability to implement a necessary feature in the future.
From a testing point of view, until a feature is actually needed it is difficult to define what it should do and how it should be tested. It might not even be needed at all or the actual feature needed might be completely different to what you implemented.
Then you have the worst case, adding the new feature leads to thinking of more new features that should be added, which are then added, creating a snowball effect also referred to as creeping featurism.
One of the benefits of only implementing new features when you need them is that your code remains simpler and simpler code is easier to maintain and refactor.
Remember that if you need to implement the feature in the future, provided you are following the appropriate practices, it should be just as easy to add it in the future when it is needed as it would be to add it now when it isn’t needed.
References
http://www.artima.com/weblogs/viewpost.jsp?thread=331531
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