I like the idea. I can't think of a specific use-case for me yet, but email integrated apps are often pretty cool and make for easier adoption.
1. Ad hoc image album. Put a bunch of images as attachments in an email and send to p@mailp.in to create a simple page.
2. Forward company receipts to p@mailp.in and create a page, and share the link in coworkers ... (this sounds weird ... but it works well for us ...)
3. Forward marketing spams to create page, and publicly share such pages to denounce those spammers :)
4. Forward paper acceptance latters or other letters to create pages, and share them to show off -- vanity!
5. Ad hoc blogging ... Don't want to have an account, don't want people to follow me. But occasionally, we still want to say something and share to Friends that we really trust.
6. Create a playlist of youtube videos. Put a bunch of youtube links and send to p@mailp.in. A page with embedded videos will be created. We don't have youtube accounts ... sorry ... but still want to share a bunch of videos with friends ...
7. Tweet more than 140 characters. Create a page with short url, and tweet!
There would be more use cases. The point is, we want something dead simple. No registration, no app (well, email client is still needed ...), and private.