I know that solr.xml is due to be replaced by solr.properties soon, so I will say solr.* and you can use whatever extension makes sense.
There is a small but very important amount of information in solr.* that doesn't specifically have to do with the cores that are local to that server. With the advent of SolrCloud, the amount of such "global" information has grown, though it is still relatively small.
If you want to change these "global" options (or you have config files in git/svn), you can't just copy solr.* from one system to another, because that's where cores specific to that server are defined.
I would like to continue to have these options work if they are in solr.*, but have an additional file for global options, with a filename prefix like global, solrglobal, globalsolr, solrcommon, ... whatever bikeshedding comes up with. That way you could put zkHost, lib, and other things that will be common to all servers in the new file, and put machine-specific things like host and port in solr.*. Any setting in solr.* would replace the global setting, so you could put port in either file.