How is this remotely trending? Yes, HN, let's vote a MITM attack to the frontpage. This is a terrible idea.
Yes it allows you to do back stuff. But it also allows great things as mashups, retrieving external content easily etc.
You're asking that I trust you will not modify the response in any way shape or form. The very nature of software dev mandates that I do not trust data that is not my own; and even then, verify it. You're asking quite a lot here. This is by all points and purposes, a man-in-the-middle attack vector.