Having different variables under the same name, especially when we have to keep track of which is which (different values in different contexts, defined by indentation) does not sound like a good idea to me.

For example, here's a problem I had in C when I used a local variable with the same name as a global variable: http://stackoverflow.com/questions/19214293/unable-to-modify...

You're saying: "immutable things tends to be easier to reason about because names do not change what they refer to within a context" Can you please provide an example of that? Maybe some code where this 'feature' is useful? Your example of let x = 3 in print ((let x = 4 in x) + x) shows what you mean, but it does not show why would I want to do that.

Thank you very much for your effort to help me understand.