Mandatory auditor rotation is designed to address a potential conflict of interest between a public company and its auditor. Because an auditor is hired and paid by the public company it audits the auditor's desire to maintain a good relationship with its client could conflict with its duty to rigorously question the client's financial statements.
If you're not willing to work hard let someone else do it. I'd rather be with someone who does a horrible job but gives 110% than with someone who does a good job and gives 60%.
