These threads rarely get into the most interesting aspect of the "how many strings are enough" question:
What sort of gigs is it for?
I spent the first 15 years of my career doing what we now know as casuals- pickup gigs out of the union book, subbing, theatre work, etc. Lots of covers, a fair amount of original music, folk, ethnic, jazz, prog rock, top 40, theatre music...
The most important thing for that work was to be ready for anything:
- any style
- any key
- any time signature
- any tempo
- any type of notation, or none at all
I was pretty successful, with nothing more than a decent amp, a P-bass and a couple of pedals.
But it was hard work, particularly when you had to cover stuff in odd key signatures, and try to keep the essence of a barely-remembered riff or an unfamiliar tune.
My job got a lot easier in the 16th year, when I got my first 5-string. All of a sudden I had some great new tools:
- Notes below the low E, that I could hit without having to detune or use a hipshot
- more ways to get out of a tight spot when soloing up in the dusty zone
- the ability to play wider intervals without sliding
- easier transposing into odd keys
I realize that none of this stuff is relevant if you only play familiar material, generally only play any given song in one key, play in a guitar band, or always stick to simple lines.
But for me, that first 5-string was a revelation, and in the last 30 years, I've never been without a 5'er in the house.
The last few years, I've been mostly playing material that could just as easily be played on a 4-string. Sure enough, just as I was starting to relax, I lucked into a couple of new and challenging situations, where the 5-string makes perfect sense again.
If you don't need it, you don't need it.
But some of us do.