I ask this question after receiving hundreds of business cards from attending an event in Atlanta GA called Industry Day. As I was going through my pile of cards to follow up, I realized I couldn't get in touch with close to one third of the people who ASKED to be contacted! Why? Their business card either had no e-mail address, no website or was in another language...I kid you not.
Bottom line, business cards are meant to make a connection, to allow others to know a small piece of information about you and your business to ultimately make a relationship and hopefully, a sale somewhere down the road. If your new contact has no idea how to get in touch with you, you have just closed all roads to making a potential business relationship in any form or fashion.
If you're on Active Rain, you know the power of networking. Don't think that power doesn't exist outside the internet. Good old fashion hand shaking and card exchanging may be "old school" but it works :-)

A very timely reminder...I agree that a biz card is about an easy contact, and not about "beauty".
Tara,
I'm guilty of not having my email address on my cards. I paid a company to design them and didn't even realize it. My website and my phone number on them but until someone pointed it out I hadn't noticed.
On my next set I will be adding my blog site and my facebook account.
I'm thinking of printing a card that says
My name is Lane Bailey
Google me...
It would certainly make people curious...
Lane- Brad Carroll did that from Dakno at Inman in NY this year. Quite clever and talk about inspiration to keep up on your SEO results! :-)
I know a few people have done something similar... And it would make you work hard to control your message...
It is important that you add email address as well as your networking account maybe. People find it easy to recognize you if you have a complete and well stated profile.