Stronger, better, faster ads
SPDY header compression copes with massive cookie bloating. Those massive cookies generally come from third party advertisers and tracking services, like Google. Many people may find that blocking excessive advertising and tracking cookies is a much simpler and effective solution than an advertiser-friendly protocol.
SPDY multiplexing is similar to HTTP pipelining in efficiency. Unless you're on IE, you're probably using it now. SPDY multiplexing can prioritize/reorder responses but I don't see many web sites getting a feature that complex correct. It sounds like a way to adjust the priority of advertisements loading rather than a way to control content loading.