Personally as of yet I have failed to observe it having a positive effect on me or anyone else that could not have been done better without fear (through rational means)
Fear is useful, or rather part of fear is useful. The cocktail of chemicals pumped into your body for the flight/fight/freeze response is good for what it was designed for, real danger. Tunnel vision, blood being pulled away from extremities, loss of control of the bowels etc etc, all useful if being preyed upon by something higher in the food chain. i.e. physical cave man stuff.
Not so useful for driving tests, meetings, interviews etc i.e. social situations in the modern world.