There can be no true utility in anything that is false. Only deception.
This is the import of what both Mr. Cherry and myself are trying to convey.
Christianity is true or it is false. It is "either or", not "both and".
Attachment is definently a reason why one may hold to a view even after it has been shown to be false, but is not justification for holding to such view. It is an excuse justified by the mind of its proponent at the behest of the heart in the face of truth and despite of the truth. This clearly is demonstrative of one's disregard for truth in favor of a lie constructed to coalesce with one's own fancy.
You do quite well in your presentation of a case to support your view, but upon closer inspection, we see it fails to convince. For if a Muslim is presented with evidence that Islam is false, he would not be truly rational at all in seeking to resist the abandonment of his faith in pursuit of truth. In his mind he may be convinced that grasping to a lie is rational or expedient for him, but it is merely a matter of self-deception.
There is no true utility in that which is false because that which is false does not correspond to the actual reality in which said proponent of that which is false resides.