The middle mouse button (MMB) does many thing, some contrary to others.
In many situations it means done or move to the next step in the process. File > Open > select a file > MMB opens it. MMB selects the check mark in the dashboard, completing the feature.
In others, however it does the opposite. In the Component repeat dialog it actually will cancel, removing all the placed components.
In other situations it starts a new process. In the view manager it creates a new view, simplified rep, explode state, etc. When placing notes or balloons in a drawing it starts a new note or balloon placement.
During symbol placement it does the same as the LMB - places a symbol.
In others it does nothing at all.
Since I mostly associate it with 'done' or 'next' I cant tell you how many times I've accidentally created a new model orientation in the view manager or started a new note or placed an additional symbol or canceled and lost all the new components I created during the repeat command.
As a primary control the function should have high level consistency throughout the software.
Since in the majority of situations it is used for next or done, I strongly feel that is the most logical choice, however, consistency in function is more important than what the actual function is. As an option to help users who feel specific old MMB behavior was more desirable, MMB config.pro options could be created to restore prior functionality in the areas that would change.