All depends on what you mean by "providing."

If it's "Hi, I'm DenverBrian on FT, and here's my ID so I can get into your secure facility," I think that's fine.

If it's "here are forms for you to sign DenverBrian where we insist you list your FT handle, actual name, and SkyMiles number so we have a permanent record which we get to use in any manner we see fit..." I got a problem with that.

If it's "if you'll allow us to match your FT handle with your SkyMiles number we'll deposit 100,000 miles in your SkyMiles account and make you lifetime PM..." Where do I sign up?