If you are an experienced rider with a well established riding posture, then I don't see much value in them. Then again if you want a different posture for commuting or whatever than you want on tour maybe it makes sense. Personally I never change my stem and my road and touring bike are set up the same.
My companions on the TA were newish cyclists and an adjustable stem was great for them. They started with it high and gradually lowered more and more it as they got acclimated to riding. By the middle of the country I think they were pretty set in what they needed and left it alone from then on.