Plastic is not always necessary:

Coded magnetic strips are increasingly used on paper tickets used by airlines, trains, and other forms of transportation, as well as those used for admission to trade shows and entertainment events.

In addition to data storage, KURZ foils can be used for counterfeit-proofing.
This is because our machines not only apply conventional magnetic foils, but can also be used to adhere continuous holographic strips containing optically variable security devices (OVDs).  OVDs are based on diffractive structures incorporated into the foil layers. 

Depending on the illumination and angle, these appear as varying colors, patterns, and designs.  The complexity of the designs, as well as the difficulty required to reproduce them, effectively deter color-copied counterfeits of paper tickets protected in this way.