Well, the big reason is enhanced usability. There are only a handful of DAV servers and even if they do change the address and don’t let you know, the long address will still work.
This would be true if there would be no better methods to detect the DAV paths – but there are And these will be implemented.
can in owncloud maybe .htaccess rewrite-engine be used to follow up the standard?
Indeed, see http://doc.owncloud.org/server/5.0/user_manual/pim/troubleshooting.html
(Actually, it’s not “Troubleshooting”.)