The usefulness of the c finger becomes really evident if one, as I did, develops focal dystonia in the i finger. In my case, my thumb and index fingers on the right hand became smeared or confused on the cellular level, and are no longer fully independent of each other. While I explored Postlewaite's books many years ago, I found that working with p-m-a-c became a way back into playing some classical repertoire and also playing chord accompaniments in my jazz and Latin work. The supinated posture that brings the c finger closer to the strings can produce great tone if you do your nails to accommodate it, as Thomas Viloteau demonstrates on tonebase. I now incorporate p-i-m-a-c-a-m-i arpeggios in my daily practice, as well as chord studies using 4 and 5 strings. This has had the effect of balancing the hand and improving the relationship between p and i, thus effecting a "cure", little by little, of my dystonia. In any event, the use of the c finger is an improvement in efficiency, if nothing else.
Lester Devoe Flamenco Negra
Lester Devoe Flamenco Blanca
Aparicio Flamenco Blanca with RMC pickup
Bartolex 7-string with RMC pickup
Giannini 7-string with Shadow pickup
Sal Pace 7-string archtop