Where you have pooled finances with another person you have created a combined estate and it is important that you jointly decide how best to leave that combined estate in a way that takes account of both of your wishes. Failing to do so adequately will no doubt result in the surviving spouse altering their Will after the first death and going against the mutual agreement.
Another reason is that families are so much more blended in today’s society with many families consisting of second marriages, step relatives, adoptions etc. So consider these 2 scenarios by way of example:
Husband and Wife are both second marriages. H has 2 sons from a previous marriage and W has 2 daughters. They separately prepare Wills leaving their half of the combined estate to their own respective children. However, they share a matrimonial home. H dies leaving W to now own half of her longstanding matrimonial home. H’s half now belongs to his sons and his sons want the house selling asap….
Had they prepared mirror Wills they could have provided for each other first and agreed on how to divide their estate between their children on the second death.
Husband and Wife are both second marriages. H has 2 sons from a previous marriage and W has 2 daughters. They prepare Wills whereby:
H leaves everything to W if she survives him otherwise he leaves his half of the estate to his 2 sons equally.
W leaves everything to H if he survives him otherwise she leaves her half of the estate to her 2 daughters equally.
H dies first.
W has now inherited the whole value of the estate and on her death, the whole estate passes to her 2 daughters equally thus disinheriting H’s 2 sons.
If W had have died first her daughters would instead have been disinherited.