Pros: Sounds like nothing else. And you can play notes with it that would sound out of place or harmony with a regular bass (that is why JPJ likes multi-string basses).
Cons: Not everything that sounds good on a 4 string works on an eight-string. It pretty much restricts a bassplayer in the way a 12-string restricts a guitarist (how many solos on a 12 string have you heard?, I remember one, from John McLaughlin, everybody else uses it as a rhythm guitar). You can't be really rhythmic in a subtle way with it either (not for reggae or funk players then), bending is impossible and that very much "there" sound can grate a little after a while. In a band with two guitars, an 8-string can begin to sound cluttered pretty quickly and due to the octave-strings' prominence or blurriness, it doesn't seal the bottom of the frequency spectrum as much as a 4 string does. Turn down the treble to make the octacve strings less prominent and the sound has less focus than any four string. Though it is probably only a trick on the ears, a real four string sounds deeper. Also, an eight string won't have the sustain of a four string in the middle and upper registers, something in the way regular and octave strings interact I guess, it always has a percussive note to it due to the prominent attack.
And while there are exceptions to the rule, an eight string really isn't for finger players. Only a pick will bring out the doubled octave characteristic.
From what I've heard of Mark's playing in the Nasty Habits, his playing should adapt quite easily to the eight-string and I can imagine it sounding pretty stellar on something like White Wedding. If the guitarist lets him, that is!!!
