One word.... Grunt!
I bought one to finish some demos, figuring I could double some of the bass riffs more easily. As it turned out I ended up ditching the idea as that approach made the songs heavier than I wanted, I mean instantly 'bloody' heavy.... which is not what we're really after. It was fun though, and If I were a younger man, doing metal type stuff I'd be all over it.
I'm moving the guitar on, as I can't imagine playing stuff that hardcore at my age, and I've established that it's not for our current project, but I had some fun. Really heavy, fast Low powerchord riffs, without the need to tune down would have been really handy over the years in some bands, and I can't imagine them disappearing for that reason.
Oddly enough, it's believed that the first use of a 7 string guitar on a rock record was actually Steve Vai, on Whitesnake's Slip of the Tongue, with a prototype he subsequently toured with, and there's 7 string on his solo material as far back as Passion and Warfare... so as (phenomenally) adept as he is on 6 strings, he's not opposed to a bit of 7 here and there. As with the age old, incredibly dull, 'Jaco Pastorius only needed 4 strings' crap that keeps coming up in the bass world (to which the obvious retort is 'Bottesini only needed three strings, and everything he wrote makes Jaco's work look like a 5 year old painting by numbers')... It's art, and it depends entirely on the sound you want to make.