This would break uses of locals(), e.g.

def foo(a, b):
    x = a + b
    if not x:
        return None
    del x
    print('{x}, {a}, {b}'.format(**locals()))
    return a * b

foo(1, 2)

Plus if the calculation raises an exception and I'm looking at the report on Sentry, I'd like to see the values of all variables. In particular I might have expected the function to return early and I want to see what `x` was.