It's often argued that atheists should just shut up for a number of reasons. They hurt the cause of science education, they don't understand faith, they are misinformed, they'll make everyone turn gay and have non-stop abortions, etc. One such argument is the grandma gambit, the idea faith gives a lot of comfort to dear sweet granny and telling her would break her heart. While many take the grandma gambit and perhaps there is reason not to make Grandma worry about your eternal soul, the implications of this argument is that we should treat everyone who has religious belief as if they were old and frail.
While I think grandmas in general are tougher than we give them credit for, most people aren't dying grannies. Any time this gambit is offered it should be shot down by everyone else believers included. Because the insinuation is incredibly insulting, that people can't even handle vocal disagreement on their beliefs. Even beyond the personal insult it's a very condescending view of humanity in general.
But perhaps it's a historical warning, after all much blood has been spilt in the name of religion. But if this is the case then there's even more reason to openly discuss it while in a much more secular and safe environment. Making a stand isn't good idea during the Inquisition, but if it's the case that it there is religious fuelled violence then during a largely neutral time in history is the best time to confront it. Though I doubt most people would take this line of argument.
Rather I think the reason for the argument is nothing to do with the frailty of believers but that discussion could cause doubt among the faithful. And since their religion is the One True Faith™, this is not only going to mean that people turn away but that's at the risk of eternal torment. The consequences are much much higher than can ever be adequately fathomed by the human mind. For the same reason that the Shroud of Turin has the perception of being genuine, because despite all the evidence of it being a Middle Ages forgery if it helps bolster someone's belief in Christ as saviour then the lie by omission is justified.
That's what the Grandma Gambit is designed to do, it's designed to shut down debate. It's a weak case but in a society where religion is a taboo subject to begin with the argument is at least somewhat effective. And if believers who use the gambit want to engage in what essentially amounts to a pyrrhic victory, then let them being the condescending ones. Because no matter why the gambit is used the insulting condescending implications of the gambit are there for all to see.