I have four Tikkas', with the oldest being seven years old) and have never had a concern or problem with the polymer (plastic) parts. They are designed to endure heat and cold and will definitely not rust.
With respect to the box magazine: I dropped one in the dark in our camp parking area (SB2 limestone fill) and my hunting buddy ran over it with his car before I could pick it up. It did not bend, break, nor showed any signs of distress other than surface scrathing and minor indentations from the limestone fill. It fit into the rifle perfecty and functioned as good as new. If it had been metal, it would have had a better chance of being bent.
The only negative I see is the price Tikka/Beretta are asking for a spare. The over $60.00 price is too much regardless of the material of construction. I have not been tempted to pay that price, which may some day bite me in the butt for need of a spare, so I take extra care to not lose one.
Again, I have not had problems and would not hesitate to purchase any Tikka.