Sure, you can apply an update to a full clone: it just acts like another volume, so that's no problem. However, any changes you'd make while booted from that clone stay on the clone... so, there's no way to roll back while retaining your user data.
With a Safety Clone, when you roll back your user data is retained, so you don't lose things like mail, documents, or whatever else you were doing on the clone.
With a safety clone, once you've decided you're happy with the update you have two choices: boot back to the original and install, then update the safety clone, or clone from the Safety Clone back to the original volume (scary).
In all cases, your 'regular' backup would always back up the main drive, not the safety clone -- there's nothing on the Safety Clone that you can't "get back" easily, since it's just a copy of the main drive's OS with whatever updates you've applied. Your data still resides on the main drive. So, yes -- you'd want *both* a full backup and a safety clone.
Make sense?
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--Dave Nanian
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