I am constantly using 'Insert' because the default behavior for the windows command-line is 'overwrite' mode
I have only ever had to use the 'Scroll lock' key one time...
I used to have an IBM 200MHz motherboard that had a bug (tell me if you've heard this one before...)
The CPU would not run full speed unless the keyboard buffer was full.
Scroll Lock did the trick.
Anyways, that happened exactly one time in my entire time using PC keyboards. (I notice this poll is biased towards them, I wonder what the 'useless' keys are on Mac keyboards, or other keyboards in general?)