For my needs, assigning the array to a hash fails because I need ordered pairs. Further, unless there is a very unusual need, I do not tie variables. If I am passing it around, programmers will see what looks like an ordinary hash and may be surprised when it doesn't do what they expect. By using a class, it's clear to the maintenance programmer that something else is going on.

My code also allows objects as keys and has insert_before and insert_after functionality. In short, there's a number of other things which makes it preferable for my needs when a tied hash just doesn't quite get there.