While walking my dog I thought of a few more points. First off its not a 7v6 because when someone dies your are already at a 5v6 making your hold of the territory worse. When someone dies its because they were in a vulnerable position near enemy territory, or in an open sightline. So counting on the rez to give you 7v6 makes no sense its like counting on winning an NHL game in a shootout instead of regular time. Plan to keep people alive in the first place. An ounce of prevention is worth a pound of cure.

Secondly Mercy can't multi task. You can Heal, buff, or shoot. Most other support characters can do 2/3 or 3/3 at all times. Its like choosing a single core processor instead of a multi core. It limits your choices to a singular line, why she is still good for beginners who struggle with Overwatch task prioritisation. In games more choice provides opportunity and flexibility.