When I wrote Go at work, this would mostly be annoying when interactively writing and running code; if I want to comment out the only line below that uses `a` and run it again, now I have to go back and change the declaration also. Once I want to to run that line again, I can't just uncomment it; I have to go back and declare it again. I would have to do stuff like this fairly often when trying to debug failed tests, so despite the fact that I'd never want to _commit_ code with an unused variable, I certainly would like to be able to _run_ it when debugging. I feel like a warning is definitely better than a hard error here; it's pretty easy in most languages with warnings to set up CI to reject unused variables, which ensures that your code in production won't have any unused variables.