Same as a fiver isn't just about a few extra notes below E, the main advantage in any bass with more strings is more notes available without position shifts, like many above have mentioned. Great for reading and visualizing scales and arpegios. Fewer shifts also means better economy of motion which means you can play faster.
Damn, I've almost talked myself into one.
Trade-offs: string spacing (hard to slap). Weight. Variety. Availability.