My bass parts often have fairly complex sections which often require rethinking when it comes to actually executing the parts, and having an extra string or two widens the possibilities, and allows a more ergonomic approach. Those six fret stretches get a bit uncomfortable after a while.
As for the role of the bass, to me it is just another tool. The extreme low end on a five string isn't always that useful, when a different instrument can achieve both the tone and clarity better. I do like to remove the low end at times, which makes it all that much more powerful when it returns. Moving up the neck, the bass has a different timbre than a guitar, and is more suited to melody lines in some scenarios. The same goes for chording and arpeggios. I don't know how much I'd like doing some of these things on a 12 string though - I'd need a bench press for my fingers!
