Yes... that's the point. For your metric counting AST nodes you need to have parsed the method. The other heuristic works on unparsed source-code. So you don't waste time parsing a method that you won't inline.
> Counting AST nodes is very quick and simple
But parsing is neither. And you need to have done that first. I think (there's a paper I'm sure but can't find it) that parse time for JS applications is very substantial.
It's not hard to think of worst-cases. Think about an enormous function that you would not want to inline. It's been JITed, or it's running in a byte code interpreter, and the AST has been discarded. In your scheme you'll have to parse it to find it's too big. Huge waste of time, if you can see it's too large from the source code.
So you can always use the size of your bytecode as an inlining heuristic (which is what Chakra does).
Of course that wouldn't work before v8 had an interpreter, but they could have used other heuristics. For example, they could have used the size of the AST as a heuristic and saved that even after the AST gets thrown away.