I'd say there are pros and cons when comparing the riddle format and a password. The Ravenclaw format always has a new riddle for each person, and it requires thinking and is unknown ahead of time, unlike the passwords, where they stay the same for a while and could be found out by someone (like how Sirius found out the password from the list Neville had made in the third book) On the other hand, anyone could be able to solve the riddle, including people who shouldn't be in the Ravenclaw room, whereas with passwords, there's no clues or anything to help you guess.
