A small part of one of Ibn Warraq's answers:
One must get away from this absurd thought that only an Islamist can criticize Islamism. It leads to an absurd situation, where only a Fascist can criticize Fascism. Of course, Islamists are the first to criticize, for example, Christianity. So we must not fall into this power scheme. We must look at the evidence and weigh the validity of the arguments. This is, in effect, throwing out charges of Islamophobia. Anytime that anyone criticizes Islam, he is considered an Islamophobe. This is just a way of silencing criticism. We must keep on criticizing.
Indeed. Read the whole thing.