In order to be fully effective, most security systems require the universal participation (or at least the absence of active opposition) of a site's personnel. If someone can simply opt out of your security mechanisms, then an attacker may be able to attack you by first attacking that exempt person's system and then attacking your site from the inside. For example, the best firewall in the world won't protect you if someone who sees it as an unreasonable burden sets up a back-door connection between your site and the Internet in order to circumvent the firewall. This can be as easy as buying a modem, obtaining free PPP or SLIP software off the Internet, and paying a few dollars a month to a local low-end Internet service provider; this is well within the price range and technical abilities of many users and managers.
Much more mundane forms of rebellion will still ruin your security. You need everybody to report strange happenings that might be security-related; you can't see everything. You need people to choose good passwords; to change them regularly; and not to give them out to their friends, relatives, and pets.
How do you get everyone to participate? Participation might be voluntary (you convince everybody that it's a good idea) or involuntary (someone with appropriate authority and power tells them to cooperate or else), or some combination of the two. Obviously, voluntary participation is strongly preferable to involuntary participation; you want folks helping you, not looking for ways to get around you. This means that you may have to work as an evangelist within your organization, selling folks on the benefits of security and convincing them that the benefits outweigh the costs.
People who are not voluntary participants will go to amazing lengths to circumvent security measures. On one voicemail system that required passwords to be changed every month, numerous people discovered that it recorded only six old passwords, and took to changing their passwords seven times in a row (in seven separate phone calls!) in order to be able to use the same password. This sort of behavior leads to an arms race (the programmers limit the number of times you can change your password), and soon numerous people are sucked into a purely internal battle. You have better things to do with your time, as do your users; it's worth spending a lot of energy to convince people to cooperate voluntarily, because you'll often spend just as much to force them, with worse side effects.