Surely though if the person/people getting married are your friends (which I presume they would be, if you got an invite to their wedding?) then you'd want to give them a present they can actually make use of?
I'm more than happy to give cash/vouchers/whatever as requested.
It's more than a little bloody stubborn, in my mind, when people insist on getting the bride and groom yet more wine glasses/naff picture frames/awful table linen etc when just because they feel that's a wedding present?
I've never given money (or vouchers) when people have asked for them. It's absurdly rude.
so it's not rude to give them something that they distinctly clearly don't want or need? ! I really don't get that attitude at all. 
Incidentally, yes, I'm getting married in a few weeks. We are broke. My family are paying for the very small wedding (under £3k for everything) We are struggling to pay our council tax, or buy food at the minute.
Yet apparently, some people, if they were invited to our wedding, would rather buy us something useless we have no need for because why???!!
Thankfully I have super friends who are insisting on giving us money. We haven't asked for it. Even when pressed, I've insisted, no present, just bring yourself. They've then said, no, don't be daft, you're broke, right, money it is and you can buy whatever you need. I'm not going to argue with their logic (lovely, lovely friends I have)