Here’s the link I should have had previously, it’s by Eric Raymond from The Magic Cauldron:
First, code written for sale is only the tip of the programming iceberg. …we’ll see shortly that there is empirical evidence that approximately 95% of code is still written in-house…
Most such in-house code is integrated with its environment in ways that make reusing or copying it very difficult. (This is true whether the environment is a business office’s set of procedures or the fuel-injection system of a combine harvester.) Thus, as the environment changes, work is continually needed to keep the software in step.
This is called `maintenance’, and any software engineer or systems analyst will tell you that it makes up the vast majority (more than 75%) of what programmers get paid to do. Accordingly, most programmer-hours are spent (and most programmer salaries are paid for) writing or maintaining in-house code that has no sale value at all-a fact the reader may readily check by examining the listings of programming jobs in any newspaper with a ‘Help Wanted’ section.
I knew I didn’t make it up ;)