But what if your chase chain carry an interrupt? For example, GNW Neji who combos off knockdown into high float with acupuncture.
If they use their charkra and you ARE faster (higher initiative), their mystery dies to the interrupt+acupuncture.
If they don't use their chakra, wanting to save it later for a cast, it get drained.
And besides, the mystery is the worst part of kisame. He have huge attack power when you have extra chakra (excellent with any chakra generator, like itachi, orochimaru, jigokudo), and can cause a acupuncture to whom he hits (almost guaranteed if you have full chakra), while not a tank, is reasonably durable thanks to his regeneration. The only thing kisame hates is clone, as that prevents his normal attacks from reaching valuable targets, which his mystery helps somewhat in clearing but might not be enough.