Leading is necessary to minimize diffs when you don't support trailing commas:

    Prelude> [1,]

    <interactive>:4:4: error: parse error on input ‘]’
    Prelude> [1]
    [1]

This means if you go multiline in the "usual" style:

    [
        1,
        2,
        3
    ]

any time you need to append a new item, you need to modify the existing last line as well:

     [
         1,
         2,
    -    3
    +    3,
    +    4
     ]

Meanwhile:

     [ 1
     , 2
     , 3
    +, 4
     ]

If you support trailing commas, you can just use one on the last line for the same effect:

     [
         1,
         2,
         3,
    +    4,
     ]

Incidentally, some crazy people put the closing symbol on the last line:

    [ 1
    , 2
    , 3 ]

That completely defeats the point:

     [ 1
     , 2
    -, 3 ]
    +, 3
    +, 4 ]

I did prefix my opinion with "IMHO". But no. Leading commas are, in my opinion, less common in life and a distracting, surprising style for the sake of fixing a syntactic language mistake. "Enforce" might have been an overly demanding word. Enforcing when there's a newline is probably more apt or simply allowing it to prevent giving leading comma folks ammunition.

something like `[hello, world]` should probably be allowed since `[hello, world,]` feels extraneous.