The other Bob had some good, concrete reasons for going lower, or at least for having five strings. On a more gut level...
If I only had a 4 string, it would be tuned BEAD. That's how I had my last one set up for a year, before getting my 5.
It's a *bass*, so for me, being able to go a little lower, and generally play with heavier strings, is essential.
On the other hand, that implies I could easily live without a G string, which isn't entirely true. The range, within a position, is so much better with a 5, and there are plenty of keys to play in that would force you to shift way up the neck if you didn't have a G.
But once you can do just a fret or two short of two octaves in one position, I don't personally feel a need for much more, and I just don't like skinny strings, or really wide fingerboards.
Some people do :-)
-Bob