[CCJester]

in general... why don't protoss players use warp prisms... i mean those things are some of the handiest tools protoss has for mid - game harass.
I watch a LOT of games... but very rarely will a protoss use that prism to, for example, drop zealots at the natural before warping in another few in the main.
I mean its still important to be sure you're trading evenly, but mid game economic damage to a zerg can be vital. Especially if its forcing a round of drones before you move out.

Noted - some protoss do you use them... but they're not common.