the 6 string guitar vs. 7 string guitar is just like the debate between 4 string vs. 5 string bass...
i for one use to be be vehemently against the 5 string bass (eventhough i own one), since i think unless you tune down and play high note riffs, that low B string doesnt have a purpose.
currently, ive been playing bass for almost a year now (i stopped playing for 5 years or so) ... and im beginning to understand the purpose of the 5 string bass. chops-wise, you know, the part where you show case your technique like alternate picking and slap and pop, the 5 string isnt very effective for someone like me, cause of factors like: entry level yung 5 string bass ko and hindi ako comfortable sa string spacing nya kaya sabit lagi yung mga slap and pop lines ko.
cover song wise, hindi din effective ang 5 string bass saken because yung mga songs i cover dont need to use the lower registers.
but when it comes to original songs ... ive found a purpose for the 5 string... yung B string maganda pala sya i sabay sa open D ng mga dropped-D riffs ng guitar since yung attack sa string nya mas matapang kaya mas lumilitaw yung segregation ng riffs. mas may purpose yung 5 string in a band setting as opposed to playing by yourself.
so, taking my experience, ganun din masabi ko sa 7 string. totoo nga na mas marami kang hindi magawa sa 7 string compared sa 6 string, so you tend to be disappointed kasi hindi mo na magawa yung lightning arpeggio sweeps mo at the same quality and speed, and at the same time, yung mga songs na cover mo, hindi kailangan ng additional range, panggulo pa minsan kasi nasasagi mo yung extra string at sumasabit ka tuloy.
but think of it in a band setting ... paano kung mababa range ng singer nyo? ... instead of taking the song sa key of E, may option ka na i take ang song sa key of D, without using the dropped D tuning, less hassle on stage, not to mention, mas may darker quality ang song kung nag bar chord ka using the 7th string as a root. or pwede din gumawa ka ng classical finger style picking using the lower octave dun sa B string.
or pwede rin kung sawa ka na sa traditional guitar solo, pwede mo taasan range ng instrument mo, and come up with a fresh sounding solo...
marami talagang possibilities ang 7 string in terms of it working in a band setting... kahit na cover songs nyo mga 6 string lang required... what if 2 guitarist kayo, yung isa naka 7 string and yung isa 6 string... then tugtog kayo ng "symphony of destruction" ... yung isa mag hold ng bar chords sa lower register ng 7 string... im sure magkarun ng ibang quality ang song nyo na hindi kayang gawin ng gadgets like octaver, since after all, iba talaga ang effect ng attack sa thicker strings.