The point being that they've designed this new protocol based on their assumptions and hunches. Same lack of rigor is present throughout the whole spec -- take a look at how much redundancy is in the prefix dictionary for instance.
Original whitepaper: http://www.chromium.org/spdy/spdy-whitepaper
Here's a good tech talk to checkout as well: http://www.youtube.com/watch?v=TNBkxA313kk
Draft 3 has also updated the dictionary, here's the research paper motivating the change: http://www.eecis.udel.edu/~amer/PEL/poc/pdf/SPDY-Fan.pdf
Pipelining can only be done for idempotent requests, (ie GET only), has head of line blocking problems, and can break proxies.
Also, its very difficult to handle errors in a pipelined situation. For example, if the 2nd request in a pipeline fails, but the server has been processing 3rd and 4th concurrently, what response does it give?
I think most engineers would agree that multiplexing over a single connection is better and less error-prone than pipelining.