In politics perception is reality. Hastert has given the perception that he is covering up for his buddies. This is really going to hurt the repbulicans this November. Hastert has handed the "Cultuer of Corruption" football back the democrats and they will run with it.
If ANY republican is found to be a crook after this, then Hastert will be seen as attempting to cover it up. And I suspect that there are a few more Republican crooks. Cunningham was just one. There are others. You can count on it.
But he had to stand up for the Constitution and the House.
The "Speech and Debate" clause goes back to the English Bill of Rights of 1689 and is a tenet in every legislature of English descent:
"That the freedom of speech and debates or proceedings in Parliament ought not to be impeached or questioned in any court or place out of Parliament".
And it's better that he stand up and look like he's doing something than leave it to the courts and look useless. Besides Pelosi could probably pull some grandstand stunt. Though I don't know what, the media would try to let her get away with anything she wanted.