None. Don't look at this emotionally. Look at it logically. You have your belief in your God and your religion. You presume to be right. So, if you're right, then they're wrong. And if they are wrong, God won't hear their prayers so they are meaningless.
The again, on the odd chance they are right, wouldn't you be glad that you and your loved ones got off from eternal damnation on a technicality?
It's a win-win situation as far as I'm concerned. If the Mormons are wrong, it doesn't affect you because you didn't believe them. If they are right, then they did you a kind favor.
I think you're taking this too personally. This is to be laughed at. Unless, of course, they're right.