The rare times i play panth,i usually go BC then off-tank (almost always malmortius 2nd). I've played him in the bloodmoon game with BC/Youmuu and i felt little to no difference to my Q. Maybe it's because i encountered Yasuo and Akali who healed a lot. But then i had the possibility to use my E without being instakilled and OH LORD SAVIOUR GEESUS THEIR HEALTH GOT DELETED.
So yeah,if you wanna feel the breeze of risk being squishy in mid-game,try it. It's still a good build if you know how to play the champ.
Also,ult no damage? His base is a lot if you can hit the center,and it's a slow a slow+free stun if they think they can battle. If you are off-tank,you could also do 2-3 stuns in a duel,and even 5-6 if it's a tank duel (or noodle-fight,how people call it). W is more of a utility skill,just like Ezreal's E and Sion's E.