Every answer to this is an opinion, so take it for what it's worth, here's mine.
Taller bars give more leverage front-to-back. Good for manuals, hang 5s, etc, but less room to crouch for hops (especially for shorter riders).
Shorter bars give more room for hops, and more control for spins, tucks, or tech work. Within reason of course, you shouldn't be leaning too heavily into the bars.
As for back pain, we're only talking an inch or so in height. I don't think that's enough to really affect your back unless you have bigger problems; you're mostly leaning from the hips. Stories about back pain are probably placebo effect.
Also, I think the taller bars trend coincided with the no brakes no gyro no spacers trend, and the two actually cancelled out to keep the grips at the same height. Then things got trendy stupid and everyone wanted taller bars for no reason. What you really want to adjust is grip height vs bottom bracket, through spacers, bar height, or stem rise. Consider the above (leverage front-back vs vertical compression), your personal comfort or style, and adjust.
Remember when you could ride all day and not be sore for a week?
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