Lets say 15 minutes before your gig, you drop your laptop and your screen cracks, you can use a m/m firefire cable and plug your laptop into someone elses Mac and boot from your HDD.
I work at a computer shop, yes, you can sometimes boot from another HDD, but none of the drivers will match. You might crash before even getting into safe mode to uninstall your drivers. So its not something you can rely on.