The statement “If you can’t measure it, you can’t optimize it” really sits well with me. In my short spanned working career I have already spent time on code, which did not result in any added business value. One example of this was a large refactoring on the persistence layer of an app. Although this cleaned up the code, it did not change anything seen from a business perspective. E.g. not measurably driving the business forward. This talk by Coda Hale made me realize that if a feature/change doesn’t add any business value, then there is absolutely no reason for spending time (and your employers money) on it.
There are loads of tools that can help measure metrics of an application, and in doing that, you can easily justify or reject a feature or change. A great service for measuring metrics is Instrumental. Basically you pay for not having to fiddle around setting up a metrics solution, and can therefore start measuring your app immediately.
In short: If you can’t measure the effect of a change/feature, you are guessing on the effect it will have on you business, and most likely shouldn’t be working on it.