It's rational to believe that irrationality is an ineliminable aspect of human mentality. As the consistently superb Paul Broks puts it,
The capacity to hold rational thoughts alongside irrational intuitions is part of the mind's design. Even if we deny belief in the supernatural - in ghosts, say, or astrology - we are all inclined towards magical thinking and superstition. It's a frame of mind that one direction opens out to a dream world of myth and imagination and the other leads to practical creativity in the arts and sciences. The dark side is mental illness.
Psychologist Bruce Hood claims in his recent book Supersense, that such irrationality has evolved by natural selection, by virtue of contributing to our survival at some point in the past. This is a viable hypothesis, although, as Michael Brooks points out in his New Scientist review, not necessarily one which is supported by any solid evidence as yet. However, Hood also appears to extrude the following philosophically flimsy argument from this hypothesis:
There are good, scientific reasons why religion won't disappear...Spiritual thinking is not about being simple-minded or stupid it's about being human. We are, [Hood] suggests, "a sacred species"...Our supersense gives us sacred values, and our sacred values create taboos. Taboos, in turn, provide a means for group cohesion. "Irrationality makes our beliefs rational because these beliefs hold society together," Hood says. If hardened sceptics were to accept that irrationality is, well, rational insofar as it serves to hold societies together, that would constitute an important step toward a more tolerant and unified society.
The first error here lies in the conflation of the irrational with the sacred. There are many types of irrationality, some of which are necessary to maintain personal relationships and social cohesion, but which don't involve the religious type of sacred belief. Socially-cohesive irrationality may well involve holding certain things as sacred in the sense that they are held in great reverence, but without involving the religious notion of the sacred, which explicitly requires belief in the supernatural.
Secondly, the religiously sacred brand of irrationality is demonstrably unnecessary for social cohesion. There are, for example, numerous non-religious, professional or collegiate groups, such as doctors, trades unions, and soldiers, which are not bound together by a shared belief in the sacred or supernatural, but simply by shared interests and experiences. Moreover, the existence of socially cohesive secular European states attests to the socially superfluous nature of religiously sacred belief.
Thirdly, the religiously sacred strain of irrationality is demonstrably insufficient to promote social cohesion. For example, the notoriously religious United States is beset with much higher levels of violence and homicide than secular Europe; that's hardly a great advert for the socially cohesive power of religion.
Finally, even if it is acknowledged that there are circumstances under which religiously sacred beliefs do promote greater social cohesion, such as that to be found within the Islamic theocracies which spawned Al-Qaeda and Hezbollah, the existence of religiously-driven social cohesion seems to promote a vicious in-group/out-group mentality which leads to inter-group conflict. In this respect, it is perhaps no coincidence that the notoriously religious United States is also the notoriously war-mongering United States.