If using the Gyro as an aerobatic aid... you definitely want to be able to adjust the gain or shut it off in flight.

If you are using it to stabilize an aircraft (especially some scale models of marginally successful aircraft) You'll probably find you get a setting that does the trick and leave it.

If you are using it as a training aid... you'll want the adjustability, so you can reduce gyro effect as your skills improve.

(The training and stabilizing scenarios would also apply to devces such as the PA-2 autopilot)

The gyro (and autopilot) is a tool. Use it right, and it can greatly enhance your enjoyment of the hobby. Use it wrong so that it becomes a "crutch"... your flying skills could actually get WORSE.