I knew I wanted a 5 before I bought one. Kept landing on songs in the key of D and wishing I could go lower than D2 without drop tuning my bass. So at some point.. I determined "My next bass will be a 5 string p/j"
Didn't have much luck stumbling on that til Sire released the Sire P7 and I ordered it as soon as I knew about it and had the means to do so.
Now I'm down to one 4 string, and two 5 string basses and if i were to shop another bass it'd be another 5.
I only really play original music and I feel limited on a 4 where I don't on a 5.
That being said.. Victor Wooten can play circles around me any day of the week and for the most part you only see him with a 4 string in his hands.
I personally like having access to notes below E1, and being able to use the 5th string rather than shift positions if I'm playing around between the 5th and 10th fret if I want to hit anything between a Low E to A
A song I wrote on my 4 string P-bass, is still easier to play on that 4 string P-bass. But just about anything I've written using the 5 string, and attempt to use a 4 string, I get to points where I have to transpose notes at a higher octave and miss the 5.
My conclusion is - it doesn't hurt to have a 5 string in your arsenal, so it's there if you decide "I want to go lower" or "I wish i didn't have to shift positions so far across the fretboard".
Kind of like, it doesn't hurt for me to keep a 4 string in my arsenal that I've rarely played since picking up the 5, that I already knew I wanted before I got it.
I personally think a 5 string is the perfect platform bass.. but that's just for my own use. It took some adjustment to get used to muting the Low B string, but I can be content with either in my hands for the most part.