Peter, I don't think that's fair or accurate. Anyone observing the chain gets proof of publication of matched orders, and a filtered average over the last N blocks gives you a relatively good idea of the current price in a secure way which you can compare with what you're seeing on the wire. Additionally, such a setup does not rule out the possibility of other proof-of-publication systems being used for distributing the orders. It simply recognizes that consensus over matched trades and consensus over open orders are logically distinct, and data from one need not clog up the other.
As I pointed out on the mailing list observing the chain for matched orders tells you nothing about whether or not those orders were ever published; that is, whether or not they're entirely fake and don't represent actual market depth. It's the same logic that came up in the discussion about announce/commit sacrifices. As for "other systems", remember that I did recently publish Tree Chains specifically to act as a general purpose proof-of-publication system for all uses with highly adjustable security/scalability tradeoffs.