Standby mode's (mute key) main function is to save power, and it also locks the keypad.
I think the lock button alone is weak, because it unlocks easily in my pocket, and the phone has made calls at 3 a.m. I did not want to make myself.

So, I use doble lock hehe. First, lock button, then mute key (standby mode, which also blocks the keypad, hence the "double lock").
Sure, then you will have to exit standby mode and then unlock the phone in order to use the BB, but that way I'm almost sure the phone will not unblock by itself in my pocket, and, besides, it will save battery.