Short answers...why a 5-string? I like the extra low range and positional advantages (playing in F on 6th fret of B instead of 1st fret on E). How much do I use it? Enough to justify having it, but probably a little bit less than the E, A, and D strings overall.
Long answer...I really start playing bass seriously in the late 90's and 5-strings were pretty common back then in rock groups that I listened to so it wasn't a big deal to me. Most of my favorite bassists played 4-strings but I started to build my style around having the low B, crafting lines that wove into the rest of the bass guitar pretty seamlessly. Now, I don't play original music as much but when I do, I've got to have a 5-string....I am just so used to having it that it feels limiting to be without it. In my classic rock cover band, I typically play a 4-string though because most of the material doesn't really call for a low B and I found myself almost NEVER using it. I wanted to a more classic tone, so I got a cheap P-bass that has served me very well. However, my other band runs the gamut of old and new R&B, hip hop, soul, funk, and dance music. For that band, not having a 5 would be detrimental to most of the modern material that we play. I have also been in metal bands in the past where the low B was needed, but for me, the low B has never been only about that. For example, I especially love the impact the those low notes can have during a ballad where the mix is a bit sparse, a long, sustained low D can really fill up the room.
Bottom line...if I ever did have to limit myself to only one bass, I would never choose a 4-string for my personal playing style and the type of music at I play. Just my experience. Had I started playing in the 80s or before, I might feel completely different about it.