I guess in most cases there'll be no need as they reflect your joint wishes, but things can change over time.
For example when my Grandad died, the mirror wills made by him and and Grandma left everything to the surviving partner on the first death and on the second everything was to be split between their three children, except that their younger daughter, who was very young at the time, was to be allowed to stay in the house for as long as she wanted to and all the contents of the house went to her. This seemed right and fair when she was 17, it was her home and the other children had already left home (15 year age gap). Not so much when Grandma died 20 years later. Grandma would never change the will though because it was what her husband had wanted .
So, if you've done mirror wills do you feel an obligation to leave it as is, or will you feel free to change things if your own wishes change after your partner's death?