Easily turned around...
"Politeness is your enemy."
No, its not. Lack of education and instructions is. Making people fully aware why keeping the door open even for a colleague could be a bad thing. Many IT guys keep up an unhealthy attitude regarding their policies and users and would easily answer questions about them like: "Because we say so!". Yeah, that's a sure way to motivate your users to help you do your job. Not!
If you keep creating an "us vs. them" environment then it's in my opinion inevitable that there will be plenty of users who won't take you seriously or would rather ignore you than pay any attention to what you say to them (because all you'd say is that you know best anyway).
On top of that: weakest link anyone? If opening the door for someone else could indeed be that big of a disaster then I think you have a serious issue with single point of failure.
Helpfulness is also your enemy.
Depends. In the above scenario I'd say trying to help your users to understand instead of creating an environment as "We know best" could actually have some good results. Here's not saying that it would apply all the time, but usually these bigger issues start small.
And you're also ignoring other underlying issues here. When people feel the need to be extra helpful towards their users then isn't it possible that they realize that some procedures are actually doing more harm than good?
For example: requiring that people use an 8 digit password with all sorts of extra's to make sure it's hard to crack. Yeah, obviously some won't be able to remember that and will write it down. And sure it gets taped to the computer so that they don't risk of loosing it.
As an IT guy I can see the horror in that scenario. But as someone who can also place themselves in the role of the end user I understand perfectly well why someone would do that.
Here's another question for you: how likely is it that people would try to crack user passwords from their own terminals, especially considering that there's often a lock out threshold? Also: if the password is easy to remember then there's less chance that the user would write it down. The main area where this could become a problem is if the data got intercepted somehow or if people tried to bruteforce the actual password database. Yet that part often doesn't get as much attention than the user passwords.
I'm not making this up... plenty of organizations, where Sony is the most obvious example, had very specific polities for user passwords to make sure things were safe. Only to end up getting stored in a plain text file.
Translated: users need to remember a 10 digit password, while the servers are all open and permanently logged onto as root or administrator, simply because the server room door is locked and only a select few have the key. Sure. So basically the single point of failure has now become 1 simple, yet physical, door. Some call that security, I call that false hope and, as mentioned, a severe single point of failure.
Of course it's the users who get the most blame.