One, the ability to do point in time recovery of the database if it is needed. Having a mirror database is good for high availability but it doesn't replace the need for a good set of backups from which to recover if necessary.

Second, breaking and re-establishing the mirror is not really the best thing you can do.

Why do you want to truncate the log, or is shrinking the log what you are actually talking about. They are two entirely different things.