Before I go into the research aspect of it, there is no reason to hide the usernames from the passwords. They are already out there. The bad guys have them. So

Before I go into the research aspect of it, there is no reason to hide the usernames from the passwords. They are already out there. The bad guys have them. So why not release them so that every one can look at them? Also I am sure there are some research aspects to the usernames. At the very least behavioral deductions that can be drawn based on these combinations.