I'd also like to add what I consider to be another point in favor of an operator:

Throughout all of these related threads, I have seen many typos and misspellings of current dict merging idioms, from messing up the number of asterisks in "{**a, **b}", to even Guido(!) accidentally writing the common copy/update idiom as

d = d1.copy()
d = d1.update(d2)

in a thoughtful email... and it was then copied-and-pasted (unquoted and verbatim) by others!

I still have yet to see somebody (even those who claim to be confused by it) mess up the PEP's current definition of "+" or "+=" in this context.