Documentation is critical to business success, no matter what the business. The reality is not about "protecting" your job or keeping the PHB's from mucking with things. The reality is that it may not be what you do, but what the other guy has in his head that is critical information for your business.
In other words, what happens when a critical employee has a heart attack, or gets hit by a bus. What do you do then? If everyone has their little piece in their head, no one else benefits, and the business overall loses. Or, even more simply, what happens when someone goes on vacation? Or when you go on vacation? Do you (or does the business) suffer because there isn't a way to replicate what that person does?
In this day and age, business processes are perhaps the most valuable thing a business owns. Knowledge can be learned, information can be looked up. But utilizing information in a business is *NOT* as simple as having that information. How information is applied to the business is the key. And documenting that information is the *ONLY* way to get it out of someone's head and into the general domain.
I've had this discussion recently both as part of my business (an IT Services vendor) and as part of my customers businesses. In every case the answer is the same. The processes are the most valuable asset for any company, no matter what the size or business. In fact, the smaller the business, the more valuable, because the likelihood is that in a smaller company there are more concentrations of knowledge, more key people who, if hit by the hypothetical bus, would take with them the day to day processes that run that business.
There are many ways to approach the problem. From embedding processes into a help desk program, to external solutions such as Wiki's, to professional programs that are specifically designed to collect knowledge, flowchart it, and also align it to your business processes.
One of the products that my company handles is specifically designed for this: aligning IT processes to business processes. While this is generally a new concept, and a tough sell, when you can map out what you do in your IT department, and also see the business reasons for why you do it, not to mention see the business impact if you don't do it, the value can be staggering. This is one of the greatest untapped barriers to IT becoming part of the larger business, and demonstrating its value. Far too often IT does things because they need doing, but they don't understand how what they do affects the business, or what value their day-to-day activities actually have in the larger business. And the reverse holds true as well: the business doesn't understand how their needs and requests impact IT, or why they cannot simply "make a wish" and have things their way.
Control of your business processes is the single best way to ensure that IT is doing things the right way for the business, and to clearly demonstrate the value (monetary and otherwise) of their jobs to the PHB's and accountants and other ickey people on the business side.
Documenting processes might be the best way to protect yourself, and save yourself grief. Its only a very narrow and stupid point of view that sees this as being a way to protect themselves, and make themselves "valuable" to the business.
Bill