Grandparents have unique contributions to offer their grandchildren that could be lost when they assume the role of parents. Grandparents take over the role as parent for various reasons, putting aside their plans for themselves.
Grandparents are "special friends". They are this way because they are not in charge. Child rearing is always a challenge, but having the primary role of parent changes the "specialness" of being a grandparent.
Grandparents may feel resentment toward the child's parent's for not taking responsibility. Having feelings of failure for past parenting issues. This most likely will change the relationship with their own childen. It may even change the relationship with their spouse.
Grandparents may also experience the joys of having a second chance to raise children, the opportunity to be closer to their grandchildren and the rewards of providing a stable and loving home.