The requisite one urinal minimum you put between you and any other men who may be pissing at a row of urinals. It's a courtesy thing, and it's about proximity more than line of sight: shame shields don't obviate the need for a buffer urinal.
Of course, when
you're in a situation where the buffer is
impossible because there are more men than there are urinals (sporting events, concerts, etc.), the buffer urinal no longer applies.