ES doesn't store quite enough of the AST to recreate the function anyway. Most functions reference variables and functions defined in a lexical scope outside themselves. Getting the source with toString(), and passing that to eval(), doesn't reproduce the function, and sometimes it compiles ok but the function does the wrong thing.

Usually it doesn't matter, because these methods are used on functions that luckily only use well known names, mainly properties of window/global.

But it's a risk, and I've seen subtle bugs caused by the assumption that the function's .toString() can be run through text substitutions and eval to get back a variant of the original code.

Contrived example:

  let x = "wrong variable";

  function f() { let x = "I am x"; return function g() { return x; } } 
  f()()
  => 'I am x'
  
  eval(f().toString())
  g()
  => 'wrong variable'

Agreed. I guess the argument for it is it's more accurately reflected but... who cares? Maybe I'm not understanding what people do with [function].toString() in the real world.

Engines hold the entire source anyway, with the exception of XS, which returns "[native function]" for everything.