First? I've been playin Low B strings on 5 an 6 string basses since the early 90s. Learning how to string and set up a B string is huge. Back then, it was an art. Today, most instruments and strings are excellent. I play a 5 string exclusively now, for a lot of reasons. To avoid Downtuning my instrument for songs tuned down on records, or sitting in with bands who tune flat. Second, for more range in one position. Less looking at my fretboard. Third, to save my wrists. Songs played in first position on a 4 string's fretboard can be moved to 6th position on a 5. Less stretching, less hand fatigue. And last but not least? I love the power and authority of my B string.
But, much as I love it, it isn't always applicable in all situations. I play in an Alice and Chains tribute. Guy only played a 4 string. So I only use 4 strings. I only own 5s, and I'm not the kind of guy who runs out and buys new basses for a project. But I do approach each tune as a separate entity. I try to think in the way the original player did at time of recording, and try to match, philosophically, the intent.
On my original shows, tho, I use my B far more. It's part of my style after 30 years, and I use it naturally I don't go in thinking, "I must use my B today.", it just happens. I don't see it as a separate thing. That's my bass. I just play it. I love lows, using my B instead of low octave pedals, I can play key and sax lines, it adds freedom for me. Exactly what it was designed for. I imagine the pros you see not using their B have reason. If it isn't necessary? I leave it alone, too. But it's nice to have when I want it.
Also, some guys believe Mass equals Tone. 5 strings are a bit bigger, and that can lead to bigger tone. Plus, if only playin 4 strings of a 5, the B is a GREAT fingerstop to keep your plucking fingers from getting too far away after a pluck. When I play a 4 today, that's something I have to account for. My fingerstop is gone. Heh.