Akhil Amar and Ian Ayres suggest that Obama could nominate a Supreme Court justice before there is any indication that there will be an opening on the court, allowing for a quick replacement to occur in the event of a sudden retirement or death. Sounds fine enough, but the idea is seriously flawed.
First, the nominated person's life would be placed in limbo until a death or retirement, and the nominee may never get a chance to sit on the court if no openings occur before the next presidential election. If the nominee were a sitting judge, as most recent nominees have been, they might feel political pressure in making interim rulings--knowing the nomination could be placed in jeopardy.
Second, the pre-selected nominee might encourage a sitting justice to retire early or to hold on as long as possible. Alternatively, pre-nomination might allow for easier deal making for sitting justices to influence the selection of their replacement. The issue is debatable, but I think most would agree that a sitting justice should not have significant control over selecting their replacement.
Third, pre-selecting a nominee (in the absence of a deal where a justice has agreed to retire) is not in the interest of the President because, prior to selection, the President has the option of changing his or her intended selected replacement. In general, having an option is better than not having an option. In addition, the extended period between selection and final confirmation would increase the possibility of embarrassing revelations or events that might cause the President to retract the nomination. Retracting a nomination is distracting and difficult, causing more voter dissatisfaction than not having selecting a particular individual in the first place.
Also, pre-selecting a second nominee (before a first nominee is confirmed) may change the willingness of the Senate to consent to the appointment of the first nominee. If the second nominee is slightly preferred to the first, why not vote down the first nominee?