I had to buy a 5er for a new project about 7 or so months ago - a Bon Jovi tribute band where some of the bass lines need the extra low notes. At first I thought I could cover most of it with a drop D but, it seemed, not everything. I decided to stop using 4 strings to make myself get used to the 5s and have played 5 strings exclusively since. Seven months on, to my surprise, I find I actually prefer using the 5er to the point of selling my 'non-keeper' 4 strings.
As has been said, I can play everything I used to play on a 4 on the 5 but often with greater ease and I am enjoying this period of 'discovery' as to the best way to approach using the 5 strings. For some tunes, playing notes that used to be covered by the E string on the low B gives a more appropriate, darker 'timbre' - not always, but sometimes. Some tunes I am still in the process of 're-learning' to play in the middle of the 5er, some I kept as I used to play on 4 strings and not used the B string. I can see there are some tunes that I have played in the past that would need a drop D that can't be played easily on a 5er ('Slither' by Velvet Revolver springs to mind) but at the moment I don't play any of those.
So, what worked for me was to abandon 4 string basses and use 5 strings. Be interesting to see what happens if and when I no longer 'need' 5.