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I agree with your general analysis : it doesn't seem
harmful. I don't understand the bit about "only works when
the tuple is not being used", however. In the example script
the function 'f' stores a copy of the tuple; any number of
references can be hold accross the mutation. Here is another
example:

g = {}
def f(t): global T; T=t; g[T] = 'a'
mutatingtuple((1,2,3), f, (4,5,6)) # print T -> (1, 2, 3)
g[T] = 'b'
print g

This prints {(4, 5, 6): 'a', (4, 5, 6): 'b'}.