It's a louder world than it used to be.
"Loudness ≠ intensity" is a lesson not commonly learned these days. I've heard old bebop cats playing totally unamplified, swinging like hell with plenty of fire, but they were balanced within themselves, and the overall group volume was noticably lower than most younger groups playing the same music. I think it's a matter of "perception" within each player how loud they need to be to speak effectively. But if a horn player's blowing all out, and the drummer's playing a newer-style setup designed for volume & brightness, then the piano's gonna have to be miked, and then what choice does the bassist have? A vicious cycle, that's what it is. Then again, I've seen photos of old bands where the bass was miked, probably to run throught the house system. So using a pickup/amp was the next step, especially since horn players were being miked in clubs a long time ago. Mike one thing, pretty soon you gotta mike everything.
Plus, bassists who play w/a lower action for increased speed sacrifice fatness of tone and have to compensate when playing in situations not geared towards that type of tone. Scott LaFaro would most likely have been inaudible in a 70s-era Elvin Jones group if he had played unamped. You gotta pull the sound out of a wood, and pulling is seldom compatable with running.
I will say this - I've heard good results coming out of a little bass-specific amp called Contra. Used wisely, it reinforces the sound rather than just amping it up and (dis)color it. Of course, the better the bassist (and the better the bass), the less amplification you need, and the more you can use it simply for reinforcement.