I agree, that even though they are not required for a married couple, it is a good idea to include them.
I got one from a few family members and friends from both sides. They were simple, to the point letters each person wrote from their perspective about our relationship. How they saw changes in us as our relationship developed and also included any trips outings or visits we made with them as a couple. The ones from our parents also stated that they acknowledge our relationship, and are 100% supportive. That point was more important from my husbands side(cultural differences) then mine, but still a good thing to include I think.
To make them a little more official you can have the person writing it state their name in full, where the live and how they know/are related to you. and date and sign it.
Take Care
MadeInCanada