Yes, but what about when the self-importance is warranted?
Self-Importance isn't a toggle switch that's either 'on' or 'off.'
To almost every person on the planet, they are the most important person in their own lives*. And that makes sense - you spend more time with yourself than with any other person, and the decisions you make effect you more than the decisions any other person makes. After you, there are people like your mother, your husband, your children and your close friends who rank very high. Then there's your countrymen, or members of your church, or people you know at your favorite bar, and so on. A rank ordering merely illustrates this point, I don't mean to suggest that people actually rank-order others in terms of their importance to themselves.
Unwarranted self-importance is when you demand others rank you higher than they do. 'You should put more emphasis on meeting my needs and following my wishes than you do other people!' Now, because my father is more important to me than some random person in australia, my father can get away with acting towards me in ways that would upset me, if the random person behaved the same. So behavior that would demonstrate unwarranted self-importance in one person might not in another. I (along with members of the media) shouldn't be influenced by the whims of DM, because we are not members of his church. His followers, well... if there were any legitimacy to the church's teachings, then they should be.
The thing is, it's possible for well-meaning people to step on toes. I've demonstrated unwarranted self-importance before, to various individuals. But if I make a habit of it, and thousands of people across the globe feel I'm imposing upon them, they might organize (since the internet makes it so much easier to do so now) and work to troll me. Even then, they can only affect me insofar as I give a shit what they think about me. Unwarranted self-importance goes hand-in-hand with giving a shit what others think. DM cares that I type on this site and WWP about how short, ugly, stupid, and tyrannical he is. I don't give a shit if he talks shit about me personally. In fact, it would be USI to assume he does. That doesn't mean it would be USI for Marty, for example, to assume DM talks shit about him.
So USI isn't a toggle switch. It's a ratio of how important you really are to a given person or group, and how important you feel you are to them. If you overestimate your importance**, they'll notice and get uncomfortable with that. If you consistently do that to people across the board, you create an organized enemy group that will undermine you.
*There are organizations that will try to make you feel guilty and selfish about this, convince you to make THEM the most important thing in your life (and reward you with praise for doing so), and then exploit you. I can't think of any examples of such organizations off the top of my head though. The flaw, of course, is that self-interest and selfishness are not the same thing. I can also imagine individuals and organizations that are fiercely selfish, but act in ways that are not at all in their own self interest. Again, sadly, I can't think of an example of such an organization....
**and all there are are estimates; we don't have access to any objective measurements of how important one person is to another; for person X and Y, we have the importance X puts on Y, the importance Y puts on X, the importance X wants Y to put on them, and the importance Y wants X to put on them. Look at any relationship that just ended, any stalker relationship, any politician-constituent relationship, you'll see a variety of balances. Some work, some don't. If you are in a relationship where those numbers don't balance, you feel unhappy and may act out. If you've ever had someone point a gun at you and demand your wallet, you know what I mean - they seem awfully invested in what you think about them, and you'd rather pay them no mind at all.
edit - the popular theory that the press is evicerating the cult as a result of their investigation into the SPTimes fits with this explanation. The press knows that Scientology is small potatos. While many journalists avoid Scientology out of fear, most avoid them because it isn't as relevent to their audience's lives as, say, the health care debate. But after Scientology made it clear that they'll pursue journalists who take them on, journalists recognized the USI and attacked. If journalists are avoiding coverage of scientology out of fear, that *IS* relevent to the lives of any given journalist's audience. So they're taking them out.
But that doesn't mean the theory is true; and the fact that it's the prominent theory amongst Anons illustrates the mindset - that USI is the cardinal sin of the internet. When an organization is confronted like this, the only explanation that satisfies anons is that one side or the other has USI.