It's not really supposed to be secure against fraud by card thieves. It's supposed to deter actual cardholders from falsely repudiating their own purchases. If you chargeback a purchase as unauthorized, the retailer should be able to produce the slip with your signature on it. Then you can be asked under oath or penalty of prosecution, whether or not it is your signature. It also provides protection against errors. If you are erroneously charged multiple times for one for one transaction, the retailer will only be able to produce a single signed slip. If you actually make multiple purchases in the same amount in a row, there would be multiple, signed slips with different timestamps and distinguishably different signaturs.

There's a fiction that the retailer should compare the signature on the card to the slip, reinforced by a few large retailers with policies of checking that the card is in fact signed, but this obviously doesn't happen.