If you run any syntax checking or minification tools, this will be immediately obvious. If you test your site in IE at all, this will be immediately obvious. If you record JS errors on your site, this will soon be obvious. If you record any kind of usage statistic, this will eventually be obvious.

Take it from someone who has experimented with cutesy formatting in the past[1]: you don't need to use cutesy formatting to solve this problem.

[1 | You can end class declarations with _:0} instead of a bare } to avoid the trailing comma issue. ]

Having mechanisms in place to catch programmer errors is no excuse for adopting practices that make programmer errors more likely.

And it is important in our industry to write MAINTAINABLE code.

I freely remove, add, and move things to the bottom of my js classes without ever hitting trailing semis, or hitting a missing semi. I think that is more maintainable then the alternative

Lists usually grow on the end, not the beginning.

(I use this kind of formatting for SQL, for a similar reason.)

It is an IE bug that causes it to break. One of the many ways they do not follow the spec.

  ObjectLiteral : 
      { } 
      { PropertyNameAndValueList } 

  ObjectLiteral : 
      { } 
      { PropertyNameAndValueList } 
      { PropertyNameAndValueList , } 

Not to mention, IE8 was released half a year before ES5 (according to a quick check on Wikipedia). So yes, pre-ES5 I'd say it's a normal thing to do not to allow trailing commas.