The hardtop isn't a necessity, but if your car is a daily driver like mine, the hardtop is certainly nice in the winter. The main reason, a rear defroster.
The hardtop is nice for the previously mentioned sound deadening and keeping the salt stains off of the fabric but I think the rear defrost is the real reason for me. With 3.6L and over 325 hp you like to see who you just passed, even in the winter
Todd