Personally I would never attend a wedding without a gift, even if it was something small, but with that said, I'd not be upset if any of our guests didn't give us a gift. They are all expected to travel so therefore will all occur travel and accommodation cost and many (esp woman) will no doubt "need" a new outfit ,shoes etc, but I would however hope that they do all give us a card as it would be nice to have them all to look back on.
We will be giving every guest a thank you card regardless whether they give us a gift or not, thank you cards are not only a thank you for gifts but also to thank them for attending a celebrating the day with us.