The so-called "new atheists" (a term that I frankly loathe) take an assertion-centered view of language. If it's not an assertion about how things are, then it's mere subjective fantasy, unworthy of serious consideration, and so on.
And if that's right, then one way to advance the conversation is to open up this view of language, somehow -- perhaps by emphasizing, as James does here, the importance of metaphors and symbols.
As a "new atheist", I would object to this interpretation. If some statements sound like assertions, if they use assertion-like grammar, if millions of people seem to act as if they they believed these statements as assertions, if they hold these assertions as directly competing with scientific assertions, if they seem to construct their moral and ethical beliefs on the basis of the statements as assertions and seem eager to impose the moral and ethical beliefs on the rest of their society, then maybe, just maybe, we might be interested in discussing whether these assertions are actually true.
If McGrath wants to define religion as just a state of mind, then his religion is definitely of interest to anyone — myself included — interested in psychology, spirituality, phenomenalism, and everything else to do with the endlessly fascinating human mind.
There are hundreds of millions — perhaps billions — of people, however, who really do take statements about God, or Allah, or Shiva, or Odin or Zeus as assertions of truth in precisely the same sense that assertions about the mass of the Earth or the law of gravity are assertions of truth. Their religion is not metaphorical, it is not literary, it is not symbolic, it is taken as literally and factually true.
When we're talking about this kind of pernicious, deluded, misguided belief, then no, we're not really interested in religious people who define their religion in purely subjective, phenomenological terms.
Atheists are just as interested (and some just as uninterested) in poetry and literature as the next person. And most of us — myself included — would be well pleased and consider our efforts entirely successful if religion were commonly taken to be of the same ilk as poetry and literature.
So I get a little suspicious when someone tries to undermine the atheist critique as a whole by saying his particular brand of religion is not susceptible to the critique. Perhaps it's not; if the shoe doesn't fit, don't wear it. But why try to undermine the critique as a whole when it's patently obvious that it actually applies directly to hundreds of millions of people? This is why I suspect that those who put forth a metaphorical or literary interpretation of religion are not being entirely sincere, that somewhere under the metaphor is some assertion of truth which the believer is intent on protecting.