There are several ways that signing your debit or credit card in permanent ink help protect you:

1. Most cards state above the signature panel that the card is not valid unless signed.  A merchant can refuse to accept your card if it is not signed.

2. Thinking of writing "See ID"?  Think again.  According to Visa and MasterCard rules, "see ID" is not a valid substitute for a signature and the merchant will have the right to refuse your card.  Plus, it could actually make it easier for an identity thief who creates a piece of fake identification in your name.  Also, such a notation will not provide you with any additional protection from liability from fraudulent charges.  However, you can sign the card and write "See ID."

3. If your card is lost or stolen and the signature panel is blank, an identity thief can forge your signature on the card.

So, be sure to sign your plastic cards with permanent ink as soon as you receive them (a fine tip permanent marker works best).