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Warp prisms are both transport units and additional pylon replacements. They enable you to send out some units to distract the enemy while the prism turned into its stationary mode and allowed you to warp in units to press the attack. Combined with the ability to warp in ships and others, that'd have been awesome to fight Amon and his crew.
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Sentries already had that capability. You could also warp down a pylon in 2 seconds anywhere on the battlefield. As such, obsolete.