If you don't see solr.xml, you just assume the very simple default solr.xml - the same as solr will.
you would have to change the directory structure somewhere
Thats the point of this issue - you won't have to change the directory structure.
introduce a new configuration file, and only then you would be able to add more databases
You will only introduce a new config file if you decided to forgo it to start - thats fair. The example will come with it.
Having two deferent configuration mechanisms for single and multi-core eventually adds to the complexity
There won't be two configuration mechanisms - if you want to do core configuration you will need the file - in either case, for a release or two, solr will assume a solr.xml that works with no solr.xml in the dir - taking that away is more pain for those that don't have or want it than helpful in my opinion. There won't be two configuration mechanisms in either case.
The purpose of this issue is to make the current single core specifiable from solr.xml - that doesn't mean you need an explicit solr.xml - any tool and solr can assume the most basic solr.xml that would be the default anyway : private final static String SINGLE_CORE_CONFIG = "<solr persistent=\"false\"><cores adminPath=\"/admin/cores\"><core name=\"DEFAULT_CORE\" instanceDir=\".\" /></cores></solr>";
I'll let it go - others that don't wan't it required (from the zookeeper issue) can chime in, or I'll just let it go.