I can't tell you how many times I've had to figure out what my password and current location were two or three years ago, the last time I used X.com, because X was built by people who think they need to save more information than they do, all for the sake of my alleged convenience.
Moo and ThinkGeek are the big aggravators over the past few days, but the leader in this space is Amazon, who not only track way more info than they need, but have made logging out as difficult as possible.
The way to do this right is to have some little alert trigger itself if the same identifying info is used frequently - "You shop here all the time, would you like us to store info you frequently type?" - not to put filling out a whole unnecessary biography between choosing what you want and buying it. I shouldn't have to tell you my life story to buy a stupid USB hub that looks like a doomsday device. I shouldn't have to search through my e-mail, hoping I wrote down your UTTERLY UNNECESSARY password three years ago, either.
Honestly, online merchants, what are you thinking? How much time are we supposed to have in our day?
ThinkGeek just lost a sale because of this.
Anyway - if you are pulling this inconsiderate, time-wasting bullshit on your users, who never did anything to hurt you, at least don't require that e-mails be unique. It's way more effective for me to create a new "account" every time than to be keeping track of a password I used once during the holidays three years ago. But if, as a user, you try to go around this mess, you just get an error, that your e-mail's already in use. That's way too much frustration. "Helpful" mandatory logins are the most aggravating "features" on online merchant sites today. Implementing them should be against the law, and punishable by death.