I agree that the cards are a pretty ineffective way of dealing with the problem at events. They're only effective for people who don't know they're doing something inappropriate, but do care when told. I think that's a very small percentage of offensive people.
Selfishly, I care more about getting the (admittedly very small number of) hacker females to attend and contribute to events more than unrestrained free speech by attendees. If kicking out a few obnoxious drunken guys, and convincing others to behave reasonably, is necessary, fine. That seems a lot more effective than cards handed out by attendees. Naming and shaming people who act like idiots would also be effective I suppose, but probably more potentially open to drama and consequences than quietly ejecting them for the rest of the event.
There's something interesting about these cards - they're helping DEFCON people adopt a usable vocabulary for talking about harassment, an addition to our jargon.
Here's an example. A few days ago I was part of a Rio elevator full of DEFCON people when it stopped at a floor where a non-attendee woman was looking to get in. There was obviously no room inside, but one of the guys hollered at her to "get in anyway - you're a cute woman - we love women - this ratio needs to be improved!" She instantly disappeared. As the elevator door closed, another guy looked at him and said "dude, yellow card." Silence the rest of the way down.
It's a small thing, but I was the one DEFCON woman in the elevator, and I appreciated that verbal yellow card - in other words, while I was in the middle of mentally processing what had just happened and whether creepy attention would shift to me, a fellow attendee had quickly and effectively reset the tone of the elevator to basic civility. This is good.
Selfishly, I care more about getting the (admittedly very small number of) hacker females to attend and contribute to events more than unrestrained free speech by attendees. If kicking out a few obnoxious drunken guys, and convincing others to behave reasonably, is necessary, fine. That seems a lot more effective than cards handed out by attendees. Naming and shaming people who act like idiots would also be effective I suppose, but probably more potentially open to drama and consequences than quietly ejecting them for the rest of the event.