Reading through the Fermentis website again, I see the no-need-to-rehydrate advice applies only to the yeasts labelled "E2U" easy to use, which is most but not all of their brewing products. They are specially prepared with a protective emulsion which prevents damage and loss of viable cells during rehydration in wort, it seems; hence there is no difference in effective pitch rate between various methods including DP. So in effect, there should be no pitch rate-related differences in performance or flavor -- at least with these yeasts. Presumably rehydration in water is still advisable for other yeasts and other manufacturers' products, and the familiar, necessary assumptions about losses in direct pitching still apply there. (I hadn't noticed this was a supposedly new thing until I realized not all their strains are labelled E2U.)