When dehydrated immediately after harvesting, most mushroom varieties will retain their flavor in a concentrated form. That flavor will be partially released in rehydration liquid, but can be used to further incorporate mushroom flavor into sauces, soups, braises, and the like. In most cases, the reason you would want to choose fresh over dehydrated versions of mushrooms, it's for aesthetic and/or textural purposes. That said there are many mushrooms that "bounce back" after they are rehydrated.