Christopher Hitchens thinks that the term 'Islamofascist' is a good one; ie it is truly descriptive of the phenomenon it refers to.
The most obvious points of comparison would be these: Both movements are based on a cult of murderous violence that exalts death and destruction and despises the life of the mind. ("Death to the intellect! Long live death!" as Gen. Francisco Franco's sidekick Gonzalo Queipo de Llano so pithily phrased it.) Both are hostile to modernity (except when it comes to the pursuit of weapons), and both are bitterly nostalgic for past empires and lost glories. Both are obsessed with real and imagined "humiliations" and thirsty for revenge. Both are chronically infected with the toxin of anti-Jewish paranoia (interestingly, also, with its milder cousin, anti-Freemason paranoia). Both are inclined to leader worship and to the exclusive stress on the power of one great book. Both have a strong commitment to sexual repression—especially to the repression of any sexual "deviance"—and to its counterparts the subordination of the female and contempt for the feminine. Both despise art and literature as symptoms of degeneracy and decadence; both burn books and destroy museums and treasures.
I think there's another reason to allow the term. That is, it is a reaction to the liberal tradition that stems from the Enlightenment. Just as Communism was a sort of Enlightenment Fascism and Nazism (and Fascism itself, though to a lesser degree) an anti-Enlightenment extremism, so the thought and action covered by the term Islamofascism is a radical reaction to the world created the forces of economic and political liberalism.
It is important that we remember that. It is the product of weakness and cultural failure; it does not even present any putative alternative, which is a where it differs from Communism and Fascism. It is an expression of incapacity, one that will help neither its practitioners nor those they claim to represent.