Clone saves your system, lets you restore back so you are up and running.
If you can't boot OS X you can get started by creating an emergency OS X partition on external and use that.
Your internal main drive though needs to be returned to a functional partition for OS X and healthy partition table.
Lion Recovery or ML allow checking the health of hidden partitions.
Having a clone of your system at all times or any time makes it quick and easy to be back up and running and depending on speed and size of your hard drive and system, a full restore of OS X in an hour.
Emergency 30GB
Clone of full system 30-120GB
Windows system restore image(s) 60-200GB
Clone of home folder if desired 200GB, optional, could be just system - I just don't rely on TM only in any case
TimeMachine 350% the size of your Mac system and data (1.5TB or less)
A 2TB drive - not ideal to have all on one drive but could be one backup set.
Update of clone takes 15 minutes generally to keep it current.
The emergency system is like your own Recovery only has any other programs and utilities and full up to date of Apple OS only plus 3rd party drivers in order to function.