what potential uses could there be for a spare knob on a guitar?

MartinB

I also agree with the bass cut tone control.  I have one on my 335 copy, and I think I'd put one on any twin humbucker guitar now.  It's great if you use any sounds which are on the edge of distortion, as rolling it off really cleans things up without getting a whole lot quieter.  I used a 1M reverse log pot and a 0.0022uF cap, though people have used other values. 

Rocker

Use 'the knob' to switch the LEDs (on the fretboard) on and off.

TonyR

Rocker said:

Use 'the knob' to switch the LEDs (on the fretboard) on and off.

Was that idea inspired by my dream per chance??