Would you build a pure client-side JavaScript App using Pageforest?

I don't think so. Also, since we're at it (I took some time to read the whole frontpage) "(GET, PUT, and POST)" are standard HTTP (not HTML) methods.

I don't see any mention of database. If you can't save or retrieve records that's kind of limiting, no?

Possibly using the open source platform, fairly unlikely on a hosted version. Why? I think it's almost certain that at some point I'm going to need some configuration or tweaks to the server side code that are application specific and cannot be done in client side code (validation, heavy lifting or secret sauce things). As nice as it would be to not worry about the service side code, once you get past the basics of a CRUD app, the server side become rather important.

Off the top of my head the biggest early concern I'd have would be how are you going to handle server side validation, since relying solely on client side validation is pretty much just as good an no validation at all. Especially complex validation involving multiple fields. The next would be interacting with things like third party APIs that have access keys that need to be protected or just can't be accessed from the client (because they don't offer a JSONP interface).

I don't see any obvious benefit in using it.

Relying on the the platform to handle the user authentication and document storage is something I wouldn't be comfortable with, though I'd use it as an option (as facebook connect, google or any other authentication service out there).

In general, I think this would have been a hit 5 years ago. Now, with services like heroku, phpfog, app engine, even github having free plans, the "hosting" part seems to be solved, and something pretty similar can be done these days with some sparkle from FB Connect and a bit of rails/php for the client side storage.

Don't get me wrong, I like this kind of services because [I think] they push innovation in different directions. It's just that I think there are a few pretty well stablished and provide similar (or equivalent) functionality and I just don't see any obvious advantage between this service and the others, though I see a few drawbacks.

To walk a little further down that path, if there is database access how is validation done since none of the client code can be trusted to have been executed and all input from the client should be considered suspect?