If yer guy in back is directing you....and you see something in the mirror....and correct for it....would you not confuse the guy in back? He would be asking you to hold steady....perceive you moving left (for example) as you make a correction without telling him....he thinks it is drift....and starts his patter to correct that....would you then ignore him or move right ?
Mirrors have their use....but so does the guy in back.