Warp Prisms and chargelots regularly end games at every level of every matchup, yet not one of those units costs ANY gas (if you dont count the cost of charge). No other race has that in this game. No other race has a drop option that costs 0 gas. No other race can use said 0 gas drop unit to instantly build an entire production cycle in any location on the map.
I see the need for chargelots in a protoss army, they are essential for tanking for all the wonderful splash damage options a protoss player can entertain. I just dont see a need for zealots to also do a fair amount of damage on top of all that - especially since they require absolutely no micro. CAN you even micro a zealot? The world may never know.