Leading comma is better at resolving textual 3-way merges, e.g.:

   [ a         [ a       [ a
   , b         , b    /  , b
   , c   -->   , c   /   , c
   , d         , d  /    , dd
   ]           , e       ]
               ]

Your usual 3 way merge algorithm will correctly deduce:

   [ a
   , b
   , c
   , dd
   , e
   ]

Contrast this with trailing comma:

   [ a,         [ a,       [ a,
     b,           b,    /    b,
     c,   -->     c,   /     c,
     d            d,  /      dd
   ]              e        ]
                ]

You now have a merge conflict between "d," and "dd".

However, if you were to require a trailing comma after the final element you wouldn't have this problem.