I think people should at least send a card or a personal hand-written note if they are unable to give you a gift. To me, it's just a way of wishing the couple a lifetime of happiness. I had a few very close friends that gave us NOTHING. I kept good records of who gave us what and those that gave nothing will get the same in return. No card, no gift, nothing. Just me showing up to eat and drink for free. I guess I am just a little spitful in that way.
However, we got married June 24th of this year. One of my husband's great aunts misplaced our wedding invitation, just found it, and sent a card and a check just 2-3 weeks ago. So, some people just forget and misplace stuff. So, you might still get a few cards and gifts from people after the wedding. I know they say people have up to a year to send a gift, but I think if someone sends me one a year after I have been married I will not bother to send them a "thank you" card. It's sort of rude & tacky IMO to wait so long.
One of my friends from California said a year before I was married "my gift to you is going to be me coming to your wedding". WTF? Who says that to someone? AND, she didn't even get us a freaking card! Thanks a lot you Bi$%h!!! Those that did not even give us a card didn't get a picture of us from our wedding. Seriously, if someone can't give me something to wish me a liftime of happiness with my husband, forget you! Life is too short to surround myself with tacky people!
My husband and I ALWAYS send a gift to weddings we are invited to...even if we can not attend. It's just the polite thing to do. Or, at least send a card if nothing else (but I would never just send a card, I would at least stick $20 inside).