The issue, in my opinion, is when you want something like this:

    while m := f(many, args):
        # do stuff with m

Now, if you're writing this in Python 3.7, you often end up with some code duplication:

    m = f(many, args)
    while m:
        # do stuff with m
        m = f(many, args) # duplicate

Or something like this:

    while True:
        m = f(many, args)
        if not m:
            break
        # do stuff with m

Personally, I consider the last version to be the most elegant, but still something of an anti-pattern. So I welcome this new operator.