What?
Let mods add custom buttons on the top right button bar to open their interfaces.Why?
In vanilla, the top right bar is used for various interfaces - trains, achievements, production, etc.In vanilla, the toolbar is for tools - special items and toggles.
Currently, mod interfaces are commonly added to the toolbar while not being a tool, or to other places like the top left corner.
In terms of consistency, makes sense to have all "open some window" buttons be in the same place - the top right button bar, rather than all over the place.