Why is cross db ownership chaining on by default for system DB
I have continued to search for reasoning behind this.  Everything I find is some version of 


  These system databases must have cross-database ownership chaining turned on in order to function properly source


Clearly the system databases; master, tempdb, and msdb are interdependent. Presumably early in SQL development, some microsoft developer made the choice to use cross-database chaining to solve access issues, and it has never been updated. 

Another of of those things that is Microsoft recommends not be used, but they are dependent on it.