Hidden screens: I use them to create a helpfunction for the family members who aren't that quick with learning a new electronic toy. :)
E.g. On my home menu there is an icon called help
(thanks to Daniel for this). If pressed a screen is shown with some tips how to use the Pronto. This screen is hidden so you won't see it by scrolling through the home panels, only by pressing "help".

You can use it for several other things, they are even used in those nice games you can find here!

About the templates I'm not sure, I don't use them,
but I thought it was this:
You don't have for instance a DVD in your home theater yet, but next week you will, then you can make a template device with all the buttons and stuff for this DVD. Because it's template you want see it yet in your device list on your Pronto, but you can activate it, when your DVD arrives.

Or something like that.

Regards,
dunno