CRM 2011: Cannot unregister PlugIn

After having upgraded a CRM 4.0 installation to CRM 2011 I was attempting to unregister some of the plugins that were carried over from the 4.0 installation as part of the upgrade process. Yet whenever I tried to do so, I encountered an error telling me that the plugin could not be unregistered. The Error Message received was similar to the one shown below:

After a little bit of research, I discovered that the said plugin was actually a plugin that was developed way back using the CRM 3.0 plugin architecture and therefore had DLLs etc. in the "server\bin\assembly" sub-folder of the existing 4.0 environment being upgraded.

Once the files had been copied over to the corresponding sub-folder in the CRM 2011 upgrade environment, I was able to unregister the plugin successfully.

Not entirely sure that it makes sense in CRM 2011 to have to copy these DLLs present in order to unregister the plugin, but then again, I'm not sure it's worth spending any more time thinking about either.