Merge Modules are nothing but pre compiled libraries of components. When you add Merge module to your package and compile it, the package does not see it as a Merge module, but as a component which was added in it.
This is the same component which was added in Merge Module with the same attributes.
There is no harm in adding Merge Modules, but if you have to work for creating them, then it is like waste of time. Instead of creating new Merge Modules, you can just keep the attribute of component as Shared and if you don't want it to be deleted then you can also mark, do not remove at uninstall option (Permanent).
If there are already existing Merge Modules from Microsoft then it is a good idea to use them rather than creating your own components for them.
The use of Merge module is that you do not have to associate...