Sharing Or Keeping Knowledge?

I know we've discussed this in class but I just can't admit that I am convinced that sharing knowledge is always the best thing to do for an individual who works in a company.

I do agree that one must share if he/she works on similar tasks with other coworkers and that none of their know-how is really unique. However, consider the case of an application programmer that developed solo a program used by a credit company. He is the only person who knows how it has been coded even if the language used is known by many people. I still believe that because he knows how it has been developed, and is the only person who knows about the loopholes, or the "traps" he set in the code, he would be best to keep it for himself and become "important".

This thought occurred to me many months ago when my coworker told me his friend was in this situation. He had a huge salary because he designed it, and that the company was totally dependant on him. I know it may sound selfish, but I'd probably do the same to earn a higher salary too!