I think it's absurd to try to classify "religious fundamentalism and other forms of ideological beliefs potentially harmful to society" as mental illnesses. So many things are *potentially* harmful. With that logic (or lack of), you could classify trying to implement high ideals into society as a mental disorder because it would disrupt the current social order.
>>1 Why do you think holding strong beliefs is wrong? What we feel strongly about tells us what's most important to us. You can use strong convictions to inspire people--to do good or bad things; admittedly it's a double-edged sword.
There are people in this world whose beliefs have led them to behave harshly, or even violently, towards those who don't share their beliefs (extremists, radicals, or whatever other word you think fits). But, those people are far from being the majority. More often than not, I think people are able to reconcile their beliefs with the beliefs of others.
On a different note, as wrong as I think it is, labeling someone with a mental disorder can create huge barriers for that person to have to overcome.