I can think of one scenario where this change would be a *big* loss.
If you try to build up strings by concatenation, then the intermediate
values would have to be "unique-ified." I think that it would be very
painful (in terms of CPU utilization) to do this unique-ification on
temporaries, only to throw them away in a moment.
Bottom line: sometimes you want it, sometimes you don't. IMHO, it
would be "bad" to always force unique-ification.
Status quo in this area was probably (MHO) a very good decision on the
language designer's part.