actually, it DOES mean you are asking them to A) bring something and B) bring something specific, i.e. cash. you’re not saying “*if *you are coming, bring cash” (not that that’s very nice either). you are saying “when you come, bring cash” right along with “if you don’t bring cash, don’t bother coming”.
you are being presumptuous and rude. that is all there is to that. you can sugar coat it any which way you want, but you do come across as incredibly demanding and obnoxious. your guests haven’t asked you to host them at a $100 plate event with expensive decor at a fancy hotel- that is your choice. to expect your guests to pay for your choice is ludicrous. and if you feel like these people are doing you a favour by attending your expensive event, then why bother inviting them??
again, it isn’t a question of useful gift or useless gift, its the thought that no matter what someone brings, it is a GIFT for you, a favour. they didn’t have to bring anything, but they chose to bring something. whatever it is, you accept it graciously and move on with your life. if you don’t like it, give it to charity. this is not like going to a club and paying cover- there, you are payingfor the privilege of dancing the night away. your wedding is where you have the privilege of hosting your guests, not the other way around. no one is going to feel special because you were gracious enough to extend an invitation to your wedding and therefore feel obligated to bring a gift. it will be an honour for you if your invitation is accepted by your guests and if they choose to bring a gift, that’s a bonus.
when you go to a wedding, honestly speaking, how much thought do you put into the gift for the couple? how much cash do you give? how often do you give cash? maybe this change in re-gifting ridiculous things hence inspiring the use of this line, and instead gifting useful things whether they be cash or otherwise, will change when we decide to change our own behaviour.
my rule of thumb is, if you can’t say it to their face, don’t say it on your envelope. so if you’re the kind of person who feels comfortable enough to ring up everyone on your list and tell them that you want ONLY cash as a gift and if they were planning to bring something else, to not bother coming, then go right ahead and write it on your invites. but if you don’t have the guts to/or think its bad manners to do so face to face or over the phone, then don’t write it on your invites.