Index= Trumpet. There are valves for the different notes.
Friction=Trombone. You move the slide 'til you get there.
Over time, friction shifting becomes like playing the trombone-you just know where it is. I've been riding friction for 50 years and see no reason for index, brifters, or GASP!-Di2. There's also a helluva lot less to go wrong with friction and a helluva lot cheaper to fix when it does.
Jon