Doesn't this create the role of a payments processor? Or 'terminal' - the current strength of the system is that we all participate as potential payments processors if we generate the block. Creating a market niche for a trusted terminal provider (who would guarantee they'd get your transactions into the block chain, have up-time, not be compromised etc.) breaks away from that model.
I accept your point that you can just add the entire block chain to the download, it just seems a dead-weight. It's a question of mathematical certainty versus real world pragmatism - at some point a transaction is confirmed enough for me to send you the goods you bought - 'p' blocks... if that's enough history for us to do business - for me to risk my livelihood - then why is my standard of proof any different elsewhere?
Either way - thanks very much for replying to me. Nothing's ever as black and white as it looks at first!
Rich