What would developers want to bother sorting method categories on their own?
The desire to auto-sort them is understandable when thinking about Squeak as an eclipse-style IDE for teams of Developers.
But there is another way to think about Squeak: as a system for presenting naked-objects applications to Users. The goal of Maui is to expose as much of a Smalltalk code-model as possible, as an "application" to a User actor.
It generates a default panel for any object suitable for a User actor to interact with its behaviors. The "tabs" of this default panel are simply the message categories in the order of their Organization in the system.
So being able to see and customize that order affords the Developer actor another channel, right within her own IDE, for customizing the default panel UI.
I hope you are convinced there are use-cases for both sorting and not sorting. I don't think we can get away without a preference.