Why should anybody imagine that the current ideological makeup of the Supreme Court is/should be set in stone? George H.W. Bush nominated Clarence Thomas to replace Thurgood Marshall, for God's sake. George W. Bush nominated Samuel Alito to replace Sandra Day O'Connor. Why is it OK for the right to use nominations to ratchet the court in their direction and then deny any movement in the other direction by appeals to procedure and tradition or something?
I think they think they can get some good press out of this, still stop any nominees of Obama's they don't like that might crop up over the next two years, and can maybe ride that wave of bipartisan feeling to better results in 2016.
Because of the court's extreme ideological polarization in the last 20 years, a change of just Kennedy's seat to either the left or the right would set off a massive sea change in policy equivalent to one party winning supermajorities in congress with an ideological presidency. That huge of a change without an explicit democratic approval would arguably be illegitimate. Hence the concern. It wouldn't be a problem if the court were nonpartisan just interpreted the law as it is supposed to do, but it has become the third (and arguably most powerful) legislative branch in the current generation.