In George Packer's excellent New Yorker piece about the Senate's dysfunction, Lamar Alexander is quoted at the end offering a rebuttal:
None of the Republicans I spoke to agreed with the contention that the Senate is “broken.” Alexander claimed that he and other Republicans were exercising the moderating, thoughtful influence on legislation that the founders wanted in the Senate. “The Senate wasn’t created to be efficient,” he argued. “It was created to be inefficient.” At one of the filibuster hearings, in April, Alexander, sitting across the table from Udall, said that, for all the times the Democrats charge the Republicans with obstructing legislation, “we could say that’s the number of times the majority has tried to cut off our right to debate, our right to offer amendments, which is the essence of the Senate.”
Alexander makes two arguments here. The first is that the Senate was designed to be inefficient. That's not really the case. The political system as a whole was designed to be inefficient. That's why any bill must gain the ascent of a House elected every two years, a President elected every four years and a Senate elected every six years. The multiple veto points are the designed inefficiency of the Senate. The filibuster is not part of the design. It developed by accident -- the Constitution calls for supermajorities in a few limited instances: ratifying treaties and constitutional amendments, overriding presidential vetoes, expelling members and for impeachments.
Alexander's second argument is that the filibuster is a tool for preserving debate. This was bunk when liberals said it in 2005 and it's bunk now. As Packer shows, "debate," as most people understand the term, is almost totally nonexistsent in the Senate:
The Senate is often referred to as “the world’s greatest deliberative body.” Jeff Merkley, a freshman Democrat from Oregon, said, “That is a phrase that I wince each time I hear it, because the amount of real deliberation, in terms of exchange of ideas, is so limited.” Merkley could remember witnessing only one moment of floor debate between a Republican and a Democrat. “The memory I took with me was: ‘Wow, that’s unusual—there’s a conversation occurring in which they’re making point and counterpoint and challenging each other.’ And yet nobody else was in the chamber.”
Tom Udall, a freshman Democrat from New Mexico, could not recall seeing a senator change another senator’s mind. “You would really need a good hour or two of extensive exchange among folks that really know the issue,” he said. Instead, a senator typically gives “a prepared speech that’s already been vetted through the staff. Then another guy gets up and gives a speech on a completely different subject.” From time to time, senators of the same party carry on a colloquy—“I would be interested in the distinguished senator from Iowa’s view of the other side’s Medicare Advantage plan”—that has been scripted in advance by aides.
Here's Alexander exercising the moderating, thoughtful influence on legislation that the founders wanted in the Senate: