But as you said, it adds a lot of friction to something most companies want to be as frictionless as possible: signups. So it's a compromise. I don't expect many companies will opt for it, except, as you say, those where it's particularly important.

I stand by my reply, since my parent implied that username enumeration was rare (i.e. "every so often") and some awful security practice (i.e. "I don't even")