[roger_smith]

...

I don't have an issue regarding the popularity and usefulness of the proposed suggestion. I'm simply raising a concern about potential privacy issues.

Again, how is that more or less of a privacy issue than any other username?

The responsibility of privacy lands in the lap of the end user in this case. It's the end user's decision on whether they want to share their username or not. Everyone has their own risk appetite to consider. I simply don't consider it that much of a greater risk to privacy over any of the other already existing fields. No one is forced to complete those fields, it's entirely voluntary.