It's not unprecedented for people to have cards that aren't from their employer. They used to be called calling cards, and the idea is getting to be pretty popular again. I know lots of people that have personal "business cards," and I find them to be very helpful.
For example, I was talking to my hair dresser the other day, and he suggested that I call him regarding whatever we were talking about. His personal numbers aren't on his business card, so he whips out his personal card. Very handy for him and for me. No rummaging for a pen required.
I agree that they must be used properly - only give when asked.
I also think it's a great idea to use the back. The mini resume is tricky, though, I've seen several that have TMI. Less is more.