In the American system of government, presidential term limits restrict how long any given resident of the White House can damage this country directly. But that does not limit how long, or how much, the people he appoints to the Supreme Court can continue to damage this country, for decades after the president who appointed them is long gone.
Justice John Paul Stevens virtually destroyed the Constitution's restrictions on government officials' ability to confiscate private property in his 2005 decision in the case of "Kelo v. New London"-- 30 years after President Ford appointed him.
The biggest danger in appointing the wrong people to the Supreme Court is not just in how they might vote on some particular issues-- whether private property, abortion or whatever. The biggest danger is that they will undermine or destroy the very concept of the rule of law-- what has been called "a government of laws and not of men."
Under the American system of government, this cannot be done overnight or perhaps even during the terms in office of one president-- but it can be done. And it can be done over time by the appointees of just one president, if he gets enough appointees.
Some people say that who Barack Obama appoints to replace Justice Souter doesn't really matter, because Souter is a liberal who will probably be replaced by another liberal. But, if no one sounds the alarm now, we can end up with a series of appointees with "empathy"-- which is to say, with justices who think their job is to "relieve the distress" of particular groups, rather than to uphold the Constitution of the United States.