Q. Will I have to use MIM to take advantage of the Windows Server 2016 PIM feature?
A. No. MIM will use standard interfaces into AD to enable functionality so that privileged accounts and time-limited accounts can be used so technically you could enable similar functionality using your own code however this would require significant development work and architecture to enable the various workflows and processes to securely implement. MIM will do all this work for you so definitely will be recommended as the way to take advantage of PIM.