Is there any point to learning to play octaves with your index finger?
 

I've honestly never see someone play the root with their middle and octave with their index. All those disco players just use their index finger twice so they can hit it with their middle finger. The middle is the longer finger so it requires a slight tilt in the wrist to pull it off that way.

Are you talking about your fretting hand or your picking hand?

Yes, there is. One day you'll go to play a fill, and will have not worked out how many notes you're going to play in that fill (as you would expect). You'll come out of the fill onto a low note, and the free plucking finger you'll have at that exact moment will be your index finger. You'll duff the note and wished you spent the time practising that motion.

Picking.

I don't see why you should dwell on such things... I understand wanting to play efficiently, but just go with the flow. IMO, I wouldn't tend to use my index for the high note of the bunch, but you never know.