XTR Shadow Plus derailleur has a switch between a hard and soft chain tension. The difference is hardly noticeable when shifting.
Is there ever any good reason to use the soft setting when the hard setting keeps the chain quite more stable on bumps? What is it for?
Is there ever any good reason to use the soft setting when the hard setting keeps the chain quite more stable on bumps?
No reason to keep it off while riding. In fact it is suggested to always have it on while riding because it reduces noise, chain slap and the possibility of the chain coming off.
What is it for?
You turn it off when you need to remove the wheel, cut the chain or connect it back with a link. That's because it makes the derraileur move freely.
The switch disengages the clutch mechanism and releases the spring tension to allow for easy wheel changes. Technically this is the sole purpose of the lever and Shimano designed it simply as engage/ disengage switch and that the clutch cover had to be removed to adjust tension.
In reality riders have discovered the lever works for simple tension tuning. You can set the switch any where between soft and hard, it isn't an on and off, so you can make small adjustments to the tightness for smooth changes using the lever. If you want to increase the tension past the tightest point on the lever you need to adjust the clutch inside the clutch housing.
The disengaged setting is basically for wheel removal and installation, and not much else. As DWGKNZ pointed out, the clutch lever is adjustable between the off and on settings should you feel the need to mess with it. The only reason you really would have to not set it all the way to on is if you felt that shifting felt too "heavy" at the lever. On the other hand, if over time the clutch seems to become too loose you can adjust it further with a little more effort. Pinkbike has an excellent writeup on how to do this along with lots of pictures.
