I'm not sure it's necessary to sign up ahead of time, as with most airlines you can sign up in a few seconds online. If it's an offer for current members though, you will only hear about it or may only be eligable if you already have an account.

Lost of us, myself included, have accounts on airlines we haven't been on in many years. I have an account at Continental, have taken like one trip on CO in probably 10 years, but they have a tie in with my grocery store, so every time I buy something I get miles. I've received thousands of miles there doing nothing but grocery shopping at the same store buying the same things I would have without the miles.

So, yes there is a benefit, but it's not all that hard to just get the accounts when you hear about most offers.