Once you import an xsd file into an xml schema in the modeler, you can not export/download the xsd file from the modeler. So if you've lost the xsd file and want to make a change, you'd have to create the xsd file again.
The xsd file (or at least the schema information it contains) is available in the modeler, so it shouldn't be that difficult to create an export of the xml schema into an xsd file.
I totally agree. As workaround, you could open the MPR-file in a text editor and cut your XSD out.
Isn't the same true for wsdl files (although you can easily find these in the deployment directory)?