Stupid newbie question, but I swapped with a larger HDD quite some time ago, but never locked the drive after the swap...

What am I missing out on by not locking this drive ??

Is it only the ability to boot without the mod (for live)

Exactly, If you are running the original bios, you will need to lock the drive in order to be able to boot. Aside from running live, you would need to do this for the latest exploits as well.