Suppose you tell someone they're invited to your wedding, and they say they wish they could come but they already have another wedding that day, and so they can't.
Do you send them an invitation anyway?
I feel like there's probably a standard answer here, and I just don't know it.
Because I can make a sensible argument both ways.
Pro -- they know they would have been wanted there, and in case their plans change or the other wedding is cancelled or something else happens, they have an invitation, they know they're invited.
Con -- it seems like you're just asking for a gift.
Someone asked the same question on Yahoo Answers, I'm discovering while writing this post, and the answers are uniformly yes, send the invitation. Some people's reason is that the person might want a keepsake from your wedding even if they can't attend, which seems like a silly reason unless the person is related to you.
But if the correct answer is yes, send the invitation, it seems somewhat bizarre to send an e-mail to a friend you know can't come, saying "hey, I know you can't come to my wedding, but I want to send you an invitation anyway -- what's your address?" Or is that not bizarre at all, and doesn't sound like the gift shakedown I fear it does?