What advantages are their for using SD over TM apart from instant restart.
 

OK I know that if my machine craps out I can be up and running with SD straight away. But if I'm not too exercised about waiting for TM to restore a drive, is there any other reason why I should use SD?

It's not just time, Fred. What happens if your internal drive fails, and you don't have anywhere to restore your Time Machine backup to? Or, if your Mac dies and you need to use another while it's getting fixed... or.... (etc)