Clipless Pedal Tension

the only benefit would be less chance of you coming unclipped accidentally.  Adjusting pedal tension, does not increase or decrease float (the mount of movement side to side) it only changes the tension required to clip and unclip.

If you find yourself coming unclipped accidentally, increase tension.  If not, leave it. 

Theoretical better power transfer? Less noise in your pedal stroke? Less chance of coming unclipped accidentally. My mechanic claims that a looser pedal will increase cleat and pedal life.

Well, your chances of coming unclipped when you're exerting a lot of force on your pedal stroke are greater, which is why I keep mine fairly tight. I made the mistake once of not checking the pedal tension before I threw them on my track bike. About 20 meters in to a sprint I came unclipped and racked myself of the top tube and about biffed it up near the top of the track. Oops.

As someone who has come unclipped in the middle of a full out finish sprint at 35+ I can tell you there is tremendous benefit to having correct tension in the pedal ........

(I hadn't check it in a while and it had gotten looser over time.  It was a really scary situation but I didn't crash.  Blew my sprint obviously.  I don't use that style of pedal anymore though - not because of the unclipping).