[1_player]

It is only useful in the very niche case that the incorrect variable name you're using is defined (otherwise you'd get an unknown identifier error) and the correct name you should have been using isn't used anywhere.

Would catch this:

  var a, b
  c = a + a // whoops, should have been a + b, compiler complains about unused b

Doesn't catch this:

  var a, b
  foo(a, b)
  c = a + a // whoops, should have been a + b, but still compiles

All in all, unused variables being errors is an awful feature that isn't very helpful in practice, at the cost of making experimentation a pain in the arse.

There is nothing that kills my state of flow more than having to comment a piece of code that is unreferenced, because the compiler complains, while I'm trying to hack and explore some idea.