Drop D is not as significant as you believe it to be, nor is it "entirely different" from standard tuning. "Drop D" is merely a method for playing power chords(i.e., Root-Fifth-Octave) on the bottom 3 strings with one barred finger. The 6th string is dropped to match the fret of it's octave on the 4th string, while the tuning of the other five strings remain the same as standard. Considering Drop D tuning was popular long before Tool was formed and it was hardly what "the band has evolved around".