In your example, if you were using Firefox SSB's (or choosing isolated profiles in chrome/chromium SSB's), the Facebook and Twitter SSB's would be completely unaware of each other (running in separate instances with separate profiles) and hence more secure.
Ice allows:-
a) us to have functional 'applications' on the LiveUSB/LiveDVD without actually having to make locally pre-installed software choices for the user .. it's our ethos not to make these choices wherever possible.
(I've always hated having to uninstall applications chosen by distro devs WAY more than installing my own choices .. ICE allows having say an office suite available but without actually having anything 'installed')
b) users to add (or remove) web applications and integrate them into their system menu in as easy a way as possible, and have them 'behave' more like a local app.
c) to have isolated profiles for specific web pages allowing addons specific to that page .. for example for banking you may not want the same addons you use in your main browser, you may not want the main browser profile to remember your banking password (or even ever see it), and you may not want the banking SSB to use Firefox Sync, etc.
ICE is about offering OPTIONS .. Is it of use to everyone?, NO, but then we're not trying to force its use on anyone, if you don't like it don't use it .. it's no more for everyone than any other application/utility can be.
Example use cases...
i) Wanna give a PC to your grandparents who don't understand the dangers of the web .. why not set up an isolated SSB for their banking site, and tell them "wanna use banking, click this icon" (after maybe copying the icon from the menu to the desktop) .. the SSB will have it's own profile, run in its own window, be isolated from other browsers and SSB's, and be stupid simple to explain/understand.
ii) Personally I have a few web radio stations as SSB's .. easy to fire up, single purpose, and my main browser addons don't interfere with them.
