It would be useful if ViewFS had the ability to designate one FileSystem as a "primary"/"default" to fall back to in the case that an entry in the mount table is not found. Consider the situation when you have a mount table that looks like:
nn1 here is being used as the primary, with a specific directory 'remote' being offloaded to another namenode. This works, but if we want to add another top-level directory to nn1, we have to update all of our client-side mount tables. Merge links (HADOOP-8298) could be used to achieve this but they do not seem to be coming soon; this special case of a default FS is much simpler - try to resolve through the VFS mount table, if not found, then resolve to the defaultFS.
There is a good discussion of this at https://issues.apache.org/jira/browse/HADOOP-13055?focusedCommentId=15733822&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15733822