Reworking Tassadar's basic attack
TalonousTassadar had a psi assault like attack once upon a time, and could have gotten it back when they reworked him last. The reason he has the beam is so that he can't really stutter step. The devs wanted Tassadar to have that "slow" feeling that high Templar in SC have without lowering his speed directly. The devs are well aware of psi assault, and chose the beam intentionally.
FuentePretty much this. :)