What is the Default Resource for, and can I delete it?
New activities are per default assigned to the Default Resource. Financial values or Available Units defined on the Default Resource apply to all unassigned activities. That lets Merlin calculate expected costs and scheduling information.
Also… when restricting permissions to a project by using user-resources, Default Resource is the user-resource which allows "Guest access" to the file. To disable "Guest access", you can simply disable the 'Is User' option of the Default Resource.
That being said, no… you are not allowed to delete the Default Resource from a project.