Politicians like to brag about having passed their bills with bipartisan support. It shows that they are willing cooperate with their political opponents and that they are—at least superficially—more concerned with the good of the country than with winning partisan victories. And it's a strategy that appeals to those crucial independent swing voters. That's why it was such big news when Sen. Olympia Snowe (R-ME) crossed party lines a couple of weeks ago by voting to approve the Democrats' health-care reform package—even though the Democrats had enough votes to move it out of out of committee without her.
But although it would be nice in some ways if both parties agreed on everything—certainly it would be less contentious—nothing meaningful would ever pass Congress if we waited for everyone to agree. While there is a value in building support for a measure on both sides of the aisle, compromise can also weaken a bill so much it is hardly worth passing. And, as Greg Sargent has pointed out, while people always say they would prefer bipartisan legislation, they may not be willing to sacrifice their policy objectives to get it.
That's part of why our two party system can be so ineffective. It generally means having both parties—each somewhat dysfunctional on its own—work together. Comedian Lewis Black quipped a few years ago that the problem was that we're governed by "The Democratic Party, which is the party of no ideas, and the Republican Party, which is the party of bad ideas." And, he added, "The only thing dumber than a Republican or a Democrat is when these pricks work together."