Well, the primary reason for the existence of SPDY is encryption everywhere and speed. Something which with current HTTP/HTTPS doesn't really exist. HTTPS is still slower than HTTP and HTTP is slower than SPDY.
A lot of the speed increase comes from the use of multiplexing. So without it, it wouldn't be able to achieve most of it's goals.
A lot of the speed increase comes from the use of multiplexing. So without it, it wouldn't be able to achieve most of it's goals.