I'm writing a song that is played from open to the 23'rd fret- I don't deploy the B string, but on another song I'm writing, it would be physically impossible to play with a 4 stringer.
Depends on what you play, I've always been inspired by prog. Even my Hero Geddy used a 5'er for "lock and key" he can get away with a 4 banger because his voice is high, and metal bands can get away using a down tuned 4, but bands like Mudvayne, Periphery and Breaking Ben need a 5'er in much (not all) of their music.
I find using a 5 string opens more doors for creativity, but with 6 strings, it starts to become increasingly difficult to fret notes and stay fast, but it makes for an amazing chord and tap machine- IMHO using a 5 with a high C is more usable than a thicc 6'er (that sounds dirty...)