This is your basic nail and your basic nail head.  You should take a good look at the head if you use nails, because you will be seeing a lot of them. Nail heads tend to lift out of the hole they make, as you may have noticed when you last shoveled snow from your deck. The edge of the shovel hits the nail heads, marring the blade of the shovel which in turn mars the deck surface.

But further, screws do something nails don't: They pull. Screwing down the deck boards actually helps level out any variations in the trueness of the joist and plank, so it takes away squeeking and retards washboard effect.

I suspect that a contractor who still uses nails is set up to fire them by the truckload, and is simply reluctant to put the extra work in to do the more time-consuming but superior method of screws.