They are for losing in a few weeks.
I've never been able to keep a card in my spokes for more than a month. I see dudes with three and four, and I don't know how they keep them so long. Maybe they just keep better care of their cards.
I've had five, but they are all gone now. All except for the one I thought was nice enough not to leave in my spokes. Now that I think about it, I realize that I haven't raced in an alleycat in a couple of months, and that makes me... sentimental.