If Martin had won yesterday, giving the Democrats the potential for a 60 vote majority in the Senate (assuming Franken also wins), then the Republicans would be able to claim they got railroaded every time a piece of legislation passed by a party-line vote. And if the Democrats could not get cloture on a particular bill even with a 60 vote majority (for example if one Democrat defected and voted against cloture), then the Republicans would be able to claim that the Democrats can't even muster the discipline to pass their agenda even with a supposedly filibuster-proof majority, and the Democrats would look weak. Either way the Republicans could try to use the situation to their advantage.
With less than 60 votes, the Democrats now need to get a couple of Republicans to vote to cut off debate and allow a vote on their legislation. If they have to work to get a couple of Republican votes, so much the better. That will make their achievements look more bi-partisan. And if the Republicans stick together and block too many pieces of legislation, then the Republicans will look like obstructionists, and their popularity will sink even further.
We do not have a parliamentary system, and the Senate was not designed to work like the House of Commons, where all the opposition can do is shout at the Prime Minister. We have a system that is supposed to give the minority some real power. So if the people of Georgia voted with their eyes open to maintain the power of the minority to filibuster legislation, well then maybe all we should say is hooray for democracy!
It is also worth remembering that there is no magic number that will always allow the majority to get what they want. Some Democrats will vote with the Republicans on some issues. Some Republicans will vote with the Democrats on other issues. So the shape and size of the majority will always vary depending on the bill that is up for consideration, and the power of the minority to filibuster will also increase or diminish accordingly.
Once again, of course, our president-elect has shown how smart he is by refusing to go to Georgia and campaign for Martin. He must have made a political calculation that Martin would probably lose, and his expenditure of political capital to support a losing candidate would only make him look vulnerable. But maybe Obama would also just as soon be forced to have to get some Republican support for his agenda. He can't very well say he is governing in a post-partisan way if he only needs the support of Democrats in Congress.