There is absolutely no reason to rehydrate, other than the fact that the rehydration process (in water) allows the dry yeast to re-build cell walls and to absorb the nutrient built into the drying process prior to the metabolic stress of the wort sugars being introduced intracellularly. The viability is nearly doubled by rehydrating with water. But, the number of cells contained in the typical sachet is well over the number needed to properly pitch almost all beers that I make, other than the occasional big beer. Reusing dry yeast is somewhat trickier, I tend to pitch almost a third of a yeast cake for ales and almost half a yeast cake for lagers (harvesting the yeast cake at about up to a month from original pitch date). My results have been great and as said by all above, ultimately, there is no need to do any of the extra effort; but in the end, I like knowing before I pitch that I have a viable yeast being pitched, even though that is rarely an issue with dry yeast.