Would if be going too far to say that those who have had mystical experiences are in very much the position of sighted people trying to explain color to the blind, or music lovers trying to explain why a piece moves them so much to someone who is tone deaf?Note the shifting of the frame: McGrath talks about trying to explain the subjective experience of color or music, but atheists are asking a much simpler question: do these experiences regardless of their subjective character compel belief in some underlying reality?
*(h/t to Larry Moran)
A sighted person can convince a blind person of the existence of color without having to somehow communicate what it's "like" to see colors. It's not only possible to convince a blind person that color exists, it's trivially easy. Scientists have been discerning the difference between subjective experiences that compel belief in various real phenomena and those that do not. We can tell the difference between X-rays and N-rays, thermodynamics and phlogiston, biochemistry and vitalism — none of which, true or false, we have any sort of direct experience about.
The rational blind person will become interested in the subjective experience of color only after the sighted person demonstrates that color really does exist. Since it's trivially easy to demonstrate the real existence of color, we tend to take such a demonstration for granted. But just taking some proposition for granted does not elevate it to an arbitrarily held metaphysical principle.
In just the same sense we skeptical atheists will become interested in a specifically theistic subjective phenomenology of religion only after it has been adequately demonstrated that the evidence compels a belief in the real existence of theism. Make the apologetic case and then we'll consider the theology.