I play 7-string guitar, and the addition of a low A is the reason, so that I can function in the "real" bass range for an octave and a third. As a jazz player, I played the electric 7 for years, great for duo work with singers, hornplayers or even another guitar; over the past few years, I've also gone to 7 on the nylon-strung instruments, for more bass, and the ability to play full chords in the high positions. It's also useful for lute and keyboard transcriptions, and sometimes I retune to a B or C or even Bb. Brazilian and Russian guitarists commonly use the 7-string. With a good pickup system, like the RMC system in the Godin Multiac 7-string, one can function as bassist with chords, as I do in the African group I play with, and as I have done in my trio with sax and percussion. I now consider the conventional 6-string a "small" guitar.
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