I saw some commentary last week discussing the Terry Schiavo case. That situation took place ten years ago. You may recall that it involved a woman who was on life support for a very long time.
Her husband and her parents disagreed on whether or not to take her off life support. Since she left no written directive, and she couldn't communicate due to her condition, there was no way to know for sure what her wishes were.
Some of the commentary I saw was to the effect that with new medical knowledge about brain functioning, there is more hope for recovery for people who are comatose or on life support.
That may be true, but many of the clients I counsel on this subject want a living will that says they don't want to be kept on life suport, and are comfortable with executing a power of attorney that gives a family member the authority to order that life support be withheld or withdrawn.
Signing a living will is a personal decision. I don't advocate for or against it. I do advocate that you consider it and make an informed decision.