I'm curious, what are the practical applications of an 8 string? What songs/genres call for it?
I'm not knocking it, just legitimately curious.
Well, the practical application is for arrangements that can use a bit more sonic spectrum. You can read this to mean "rock" for the most part, but of course you can find an application for that sound in any style if your are tasteful and creative enough.
Multi course basses sound great in applications like a trio. You fill up more sonic space and one guitar bands sound "bigger" that way. Even more so if there are two guitars and/or keyboards.
Twelve string basses function the same way, only a bit "more." For me, there is a trade-off between sound and playability with the twelves, which is why I prefer eights. I tend to play busy when I can and anything I can do on a single course bass, I can do on an eight. Not so on a twelve. Complicated runs just don't fly on a twelve, they sound like mush.
Players who have made cool musical statements with 8s or 12s are almost always rockers.
John Entwistle and John Paul Jones did some pioneering work in their later work with the Who and Zep respectively (in the 70s double course basses were very new). Chris Squire used an eight on some of his later work with Yes. Jones still plays a 10 string (double course 5 string) on his solo tours.
Tom Petersson pioneered the 12 in Cheap Trick. He used 8s on the first album, I think, as he hadn't yet invented the 12.
Dug Pinnick made his 12 string a signature of the King's X sound.
Jeff Ament also made his 12 a signature part of Pearl Jam's sound, although he only uses it for a few songs. Remember, PJam has two guitarists so the 12string bass didn't always make sense.
Note that real rippers above used 8s. Pinnick, Ament, and Petersson are good, but without the 12 string basses, we wouldn't be talking about them. The other three, well.....
The down side to multi course basses?
Well, for one, they are harder to play. You're fretting two strings at a time so more effort is required by the fretting hand.
Two, the tone range is higher, which means that you're playing in the guitarist's range. This can can sometimes lead to sonic competition: King's X guitarist Ty Tabor often asks for Dug Pinnick to use a four string for Tabor's compositions so that the guitar sound doesn't get "crowded." (cf: keyboards, Rush)
Three, it's a complex sound so applications for "not complex" music is tricky. You can play blues patterns on an 8 or 12, but it doesn't sound like "the blues."
Four, you pretty much have to use a pick. Not really an issue for me, but if you're a fingerstyle only guy, this is a problem. Sure, there are some guys on TalkBass that will say that they can play them with their fingers, but notice this: The Pros all use picks. All of them. It doesn't matter what we brag about on TalkBass, multi course basses sound better with a pick.
All told, they are a lot of fun. As I said, I prefer 8s because they don't slow me down. A little chorus or some tasteful preamp distortion and you can turn your power trio into a big big wall of sound. Great for four or five pieces too, if your rhythm guitarist doesn't mind being turned into a sound effect.
If you're a rock player, I can recommend an 8 if you want a heavy sonic changeup.
