Making a Will is not about wealth it is about making sure that what you want to happen to your estate does happen. It gives you the opportunity to specify such things as who will administer your estate, who will care for your children and who will receive specific items of your property.
The most obvious disadvantage is the need to wait for the funds. A widow may have a real need for immediate funds on her husband's death and a survivorship clause will increase the waiting period.
As an aside it bears noting that it is advisable for married couples to have one account in joint names so that funds will be accessible to the survivor regardless of the probate process.
Probate can often be a drawn out process in any event and the existence of an emergency fund can be of great value.
If husband and wife die in circumstances where their estates may be subject to inheritance tax, a survivorship clause may also be a disadvantage. This is due to the interaction of two different statutory provisions and is something on which professional advice is required.