Reply to Steam is worth it.
If you don't have an account, just leave the password field blank.
You ain't supporting the developers that's for sure. 
And the authentication is always one-time only. There exists many alternatives to get a temporary online solution for your offline computer to authenticate your genuine copy of the game. Heck, you should be able to authenticate the game once for your account on any online computer and then play on the offline one.
Steam makes it more convenient than ever to own games. Updates, trailers, dev tools, you get everything automatically.
And the authentication is always one-time only. There exists many alternatives to get a temporary online solution for your offline computer to authenticate your genuine copy of the game. Heck, you should be able to authenticate the game once for your account on any online computer and then play on the offline one.Steam makes it more convenient than ever to own games. Updates, trailers, dev tools, you get everything automatically.
