One of the reasons SPDY uses encryption is to ensure intermediate proxies do not mishandle the new protocol. A couple of SDPY devs gave a tech talk[1] where they mentioned the problems with tunneling a new protocol (in their example websockets) through HTTP (19:11 into video). Sending websockets over HTTP on port 80 only has a 67% success rate. HTTPS over 443 had a 95% success rate. The benefit of HTTPS is that intermediates cannot see or interfere with data transfer. The disadvantage is that caching and CDNs are more difficult to use.
As an interesting side note, tunneling HTTP over a random port (in the slides port 61985) had a higher success rate than port 80 (86% vs 67%). If I remember correctly, the speakers attribute this to intermediate proxies hardcoded to handle port 80, leaving other ports free of processing.
[1] http://www.youtube.com/watch?v=TNBkxA313kk