How many situations would you need to make a copy of a dictionary and 

then update that copy and override old keys from a new dictionary?

Very frequently.

That's why we have a dict.update method, which if I remember correctly,
was introduced in Python 1.5 because people were frequently re-inventing
the same wheel:

   def update(d1, d2):
       for key in d2.keys():
           d1[key] in d2[key]


You should have a look at how many times it is used in the standard
library:

[steve@ando cpython]$ cd Lib/
[steve@ando Lib]$ grep -U "\.update[(]" *.py */*.py | wc -l
373

Now some of those are false positives (docstrings, comments, non-dicts,
etc) but that still leaves a lot of examples of wanting to override old
keys. This is a very common need. Wanting an exception if the key
already exists is, as far as I can tell, very rare.

It is true that many of the examples in the std lib involve updating an
existing dict, not creating a new one. But that's only to be expected:
since Python didn't provide an obvious functional version of update,
only an in-place version, naturally people get used to writing
in-place code.