I've been playing 5-string almost exclusively since 1996: the only time I use a four string is when I need my fur strings (so much better then gut! ) or an instrument that has a unique sound and doesn't exist with 5-strings, namely a pre-EB Stingray.
I don't really understand the controversy over the number of strings: even the most conservative musical genre in the world, classical music, has embraced the 5-strings upright bass. You will find two or more in most orchestras.
For a practical example, to answer to that particular challenge, look up "Diavolo in me" by Zucchero: the bassline is played on the B string only, for range and timbre.
Some instruments do have a floppy, undefined B-string (I have tried some Fenders that were truly bad!), but others have a very well integrated low register. I also have a 36" 6-string bass, the definition of the low end on the B string is amazing! I haven't tried a Dingwall with a 37" low B, I'm sure it's even better sounding.
I find the Eb and D very useful, Db, C and B less so, but still useful when used in the right spot, in my personal experience. When working out arrangements with artists, most of the time they will prefer low D and Eb to their octave up equivalents, because they fill up more the bottom end and there is no range conflict with other instruments.
I also dig playing up the neck on the B string, it's a very interesting alternate sound to the same notes on the first frets, exactly like the same register on the E string. Besides, it's nice to give my fretting hand a rest at long rehearsals by playing where the frets are closer together. Another obvious advantage is that I can play chords that simply aren't playable on a 4-string.
The only downside is that you have to be more careful with muting: I put my thumb perpendicular over the strings to stop the B and E from resonating, something I didn't have to do before I played a 5... but it's a good thing, because it has freed me from playing only over the pickups, where I used to anchor my right thumb!
I have nothing against people who prefer to play 4-string, but denying the musical validity of a member of the bass family does seem a bit silly...