A bit of context might help.
First of all long spiral (index) housing changes the rules. The ends meet the adjuster at a small angle, and might bite into it a bit. Then with each shift a tiny pulse of higher tension imparts a tiny torque, that can walk the adjuster around in tiny steps.
Also keep in mind that the folks at Shimano had to justify the added cost of this design, and would not have spent the dough if they felt it unnecessary.
Lastly consider that index shifting was resisted early on as more trouble than it was worth. Today, nobody would blink at having to adjust trim from time to time, back then Shimano would have gone to lengths to avoid the issue.
So maybe not necessary, but logical at time.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.