"Extra" frets and JI vs ET
Instruments- discuss specific features, luthiers, instrument problems & questions
I would add that the 1.5 and 8.5 frets come in handy if you play a lot of very old music, such as medieval and renaissance music. I have found that those genres tend to have 'accidentals'... occasional notes that pop in or out of major/minor key creating an unexpected edginess to the tune. I have also used those two frets for unexpected minor/modal notes in older traditional American fiddle tunes which can be wonderfully quirky.
But if you don't think you'll be playing any of the types of music mentioned by Dusty or myself, I'd say you can be pretty safe just getting the 6.5 and 13.5 pair of frets... those two frets will give you lots of future-proof options.
