Seth Masket highlights a social good:
Senators really do switch positions on issues just to make the president or members of the other party look bad.
Yet even if such bickering is hypocritical and convenient, there is still considerable democratic value to it. If the president and the majority party in Congress are proposing a massive overhaul of a large chunk of the economy, the public has a right to hear critiques of it. The health reform bill may well have been the best possible legislation on the topic with any real chance of passage, but that doesn't mean it was perfect, and people should know both the pluses and minuses of its features. Only the minority party has any real incentive to bring those arguments up. Similarly, only the minority party has any real incentive to investigate the president's nominees and appointees. Sure, this creates a climate of distrust, but it also has a better chance of rooting out and preventing malfeasance than bipartisan harmony does.