Hmm, it seems to me, the logic comes down to this:
4 string: Lacking the low notes. Bass players kept finding the need to tune the low E string down.
5 string: Has the low B. A logical
correction or upgrade from the 4 string limitations.
6 string: Everything that a 5 string has, plus for those who like to play chords, and often plays "melodic solos" using the high notes.
7 string: NOTHING! Just too doggone much!...You takin it too far Doc!...
JUST SAY NO!...Run while you can!

...It's never too late!...Where'd you put your wooden stake & crucifix!!...The first step to recovery is "admission"....And just remember, you can overcome this:
"YES YOU CAN!" 
GGo on and get a 7 man!!!
And Bassthumpa, don't listen to Steelpulz! He's trying to "tempt" you man! His real name is "Damien!" and his address is
666 Elm Street!

...don't fall for it man! Be Strong!!

Thus the
Logic Verdict comes down to: "Moderation" not extremes, meaning the two in the
middle, not the two on the end!
5 string/6 string
WINS!!!
Just havin fun. Get whatever floats your boat. And if you have a boat, you're probably rich. Can i borrow $13.41 cents?...i swear i'll get it back to you October 24th....i ain't lyin! - ok, ok, if not on the 24th, then i PROMISE on the 32nd!...and that's my word!