[dnanian]

No, a regular backup can't do the same thing. If you receive mail, for example, or work on a document, if you restore the backup you'll lose the changes you've made. And, if you're testing out an update, or a new OS version, you might be testing for a few days, and you'd probably want to get some work done.

3rd party apps are shared at the time of sandbox creation, but /Applications itself is not shared. A new install on the Sandbox will go onto the Sandbox volume, and thus won't be on the original drive.