The Gervais Principle, or The Office According to "The Office"(ribbonfarm.com)

    In a company, the work you do is averaged together with a lot of 
    other people's. You may not even be aware you're doing something 
    people want. Your contribution may be indirect. But the company 
    as a whole must be giving people something they want, or 
    they won't make any money. And if they are paying you x dollars 
    a year, then on average you must be contributing at least 
    x dollars a year worth of work, or the company will be spending 
    more than it makes, and will go out of business.

    I think the single biggest problem afflicting large 
    companies is the difficulty of assigning a value to 
    each person's work. For the most part they punt. In a 
    big company you get paid a fairly predictable salary 
    for working fairly hard.

    ....

    the company has no way of measuring the value of your 
    work.

    Salesmen are an exception. It's easy to measure how 
    much revenue they generate, and they're usually paid a 
    percentage of it. If a salesman wants to work harder, 
    he can just start doing it, and he will automatically 
    get paid proportionally more.