Pipes is a wonderful thing, you can do a lot with it (like consume Google Analytics emails with it to be able to make public charts by scraping the analytics XML attachments and surfacing them as public JSON: http://blogoscoped.com/archive/2008-01-17-n73.html ).
The only problem with pipes is that if you're building anything that you will depend on (rather than it being a "nice to have") then your service now wholly depends on Yahoo maintaining pipes.
The question I always fall back to is, "Do I want my service to be wholly dependent on a third party if I can avoid it?". As you can avoid it, I'd choose to write my own proxy in this instance.
