orthodoxy

After my post on travel snobs, I started thinking... what's the deal with orthodoxy? Why do we find it necessary to ensure that others believe in True Things™? I'm certainly not immune from this impulse, but when I am able to think about it rationally it always seems a little ridiculous. One's own quest for truth is natural and reasonable, and one can certainly believe in the inviolability of that truth should one choose to, but what interest do we have in what others choose to believe?

I can think of a few reasons:
1) Compassion. One truly believes that another's false belief is detrimental to him, and therefore the impulse of orthodoxy is altruistic.
2) Power. The power of a person or group depends upon another person or group holding a certain thing to be true.
3) Ontology. If a group, which is based upon a system or series of beliefs, enforces those beliefs, it will exist longer than a group that does not enforce its beliefs. Thus, orthodox groups are selected by the nature of their practice.

I think 3 is the most common followed closely by 2 with 1 somewhere in the distance. Any other ideas?