When you go to LinkedIn, you're presented with a page that tells you a little bit about the site, and gives you a simple form that you can fill out in order to register-- enter your first and last name, email address, and a password, and you're all set. Just click the great big Join Now button and you're good to go.
Below that there's a teeny tiny line of text, the smallest text on the page, that says, "Already on LinkedIn? Sign in. There's also a sign in link at the top of the page, but it's not graphically prominent in any way.
I'm thinking about the usage patterns of LinkedIn right now. How many returning users log in for every new user that signs up? 20? 50? 100? I don't know exactly, but it has to be somewhere in that range. Let's call it 50. For every person who comes to LinkedIn and signs up, 50 people have to click an extra link just to get where they want to go.
Sure, this probably gets them a slightly larger number of conversions... people who come to the front page then sign up. And people would be confused by having two forms, a login and a signup, next to each other on the page, though this has been solved before-- make the login boxes smaller and put them at the top of the page, and make the signup form prominent. Returning users know where to look, and new users aren't distracted by the little thing in the top corner.
I see this over and over in website design-- registered users have to do more work in order to sign in, even though that's by far the most frequent activity that happens from the front page.
Why do designers do this?