In the longer term, though, I've found that being "indispensable" is a really bad strategy too. Companies know full well that if one person has a hold over them, that person can use the leverage to advantage, so it's worth the company's while spending to either avoid or get themselves out of that position.

In short, if you're the only person who knows a system, you may well find the company's been working hard to get rid of that system - and you in the process.