Very old post, but I wanted to ask if you still would follow this advice?
What about those customers that purchased a long time ago, and at that time was forced to make a password. Chances are they have forgotten all about their last purchase, and now they are suddenly asked for a password when they enter their e-mail address.
What I think would be the absolutely best solution would be to use guest checkout, for both former registered customers, and new ones. But as soon as the customer has entered their e-mail address, do a quick check on it, and show a small link - "You've been here before, want to log in?"
If the customer doesn't want to log in, they can continue with the guest checkout.
On the order-confirmation / thank you - page, they should be asked: "Want to create a password, so you can log in the next time?" and then give them a password-field and a button that says: "Remember me".
Something like that...
And what would be even better - was if prestashop was able to merge a guest-purchase with a customer-account, if a registered customer checked out as a guest.