What’s the harm in letting it go on?

First, the law requires that a special counsel investigate a specified crime based on specified facts, not try to be the second coming of the Lone Ranger.

But further, the ongoing investigation saps the resources and attention of the Trump administration. If the administration cannot function, the burden of this constantly shifting investigation will give rise to a narrative that any failure was due to the Mueller diversion — that the Trump administration was stabbed in the back. That is potentially more damaging to our politics than any salaciousness that might be tossed up by Robert Mueller.