If this piece is cracked or brittle, serious water damage can occur. If you are planning to do a top installation yourself purchase any top WITH the rain rail your installation will go much smoother. If installation is to be done by a professional you can save money by using your existing rain rail if it is in good shape. The savings by not purchasing a rain rail needs to be weighed against the cost of a longer install time.
The rain rail is a plastic "rain gutter" (the cross section is shaped like the letter "U") that pop-rivets to the convertible top and every Miata has one. It is located at the base of the top below the window (you can't see it until you remove the top) and funnels the water that runs down between the top and the body of the car to drainage holes.
I got the oil cooler installed and I'm diggin' it! Thanks. (product # 10-10012)
Roadstersport II was a great purchase, easy install, and sounds great.