there are no real advantages, just a wider range. like going from a 44 key piano to a 66 or whatever. the 66 just goes a lil higher and lower. the 44 keys in the middle still play the same. some gigs require the extra range depending on what was used on the recording. (like my silly band). johnny a
Just to preface this, I am a 4-string player, but I have to totally disagree with your comment, sorry dude. The bass/guitar layout of notes is completely different from a keyboard. If they had only one string, this would be a slightly valid comment. The guitar neck layout is like having 4, 5 or 6 keyboards all tuned in 4ths and easily accessible to one hand, with another hand providing the percussive strike on the string.
I'm a 4-string player because that's what I've played for 25 years. I am a wannabe 5-string player, but haven't found too many that have a nice B string. The lower notes are very cool to have, but what really turns my crank about the B string is the ability to play a low E at the 5th fret - I do most of my playing between the 5th and 10th frets anyway, because that's where the sound is fattest, and if I can go all the way down to a low E without moving around a lot, that seems pretty darned useful.
Some basses (Yamaha maybe?) have an extension for the B saddle at the bridge so that it's a little longer than the 34" scale of the other four strings, and IIRC Sadowsky and Lakland fivers are all 35" scale, so a longer B also, as well as the other four strings. Not sure if this helps or not...? I'd have thought a shorter scale would provide more tension, but maybe not.
A buddy of mine has an Ibanez ATK 5 string that felt pretty nice, and I liked the feel of a Stingray 5 that I tired unplugged once, and there was also an entry-levelish Yamaha BB series (around $500 iirc?) that I tried that had a decent B string.