Opt-in with the likely expectation that every developer worth hiring has one.

Who’s expectation? I don’t expect developers I hire to have a GH profile. Most GH links I see on resumes are thoroughly unimpressive anyway (one or two college homework assignments and an untouched fork of some open source project).

Having a good one is a plus, but having a poor one is worse than not having one.

Agreed: as a hiring manager I check every github profile I see on a resume. I have never been anything but disappointed.