People allow, nay, insist that their religion inform their opinions on social, civil and political matters. This necessarily has an effect on everybody.
Even if someone's religion happened to point them in directions beneficial to mankind, their mere ability to hold a belief which is based on groundless hearsay and that horrible F word, indicates that they can not be trusted to use sound means to deliberately inform their opinions in AT LEAST one matter.
If someone can believe something crazy without harming the rest of us, no problem. But that's not how it works. If someone wants to demonstrate that having reviewed a great breadth and depth of "evidence" they've come to the conclusion that there is no reliable reason to believe... but they still believed... as long as they recognize that their supernatural beliefs have no place in deciding how best we may live, work and advance together, there's no problem. But that's not how it works.
Why is it important to convince a theist that he is wrong? Because it hurts us all, in small but cumulative, massively cumulative ways.