I think the main argument for having separate playing and editing
applications is to avoid confusion for the larger group of users who
just play games using Vassal rather than designing modules. This
keeps the interface for just the players simpler, which is where a
more streamlined interface is more important.
I'm with tar on this. We need to assume that if players can be confused, they will be confused. So we need to keep the Player as lean and as simple as possible.
My take on this is if a player cannot figure out how to launch the editor then he/she probably should not be creating modules.