Peggy Noonan, in today's Wall Stree Journal, make a good case in her article The Court, Like the Country, Needs Balance. She is arguing for the wisdom of letting the country choose, in the results of the next presidential election, the liberal/conservative bent of Supreme Court Justice Scalia's replacement. President Obama no doubt would love to appoint a liberal replacement (though it probably wouldn't get through the Senate). If we currently had a conservative president, that president would no doubt love to appoint a conservative replacement (and liberals would be just as outraged as conservative are now).
In truth this is a complex argument, and is related to the previous article I recommended. The courts have by now invaded every aspect of our lives, for better or worse, and the nation is in a precarious mood right now, boarding on revolution (and the chaos that revolutions almost inevitably spawn). A lame-duck appointment during this highly politicized and explosive election season is likely to make matters worse. President Obama would be wise to see that, but based on his history over the past 7 years I doubt he is that wise.