One of the reasons that social norms are important is situations like this one:
Needless to say, this is why Democratic senators were stunned yesterday when they asked Comey if the FBI was investigating Trump over his Russia ties, and Comey replied, "I would never comment on investigations — whether we have one or not — in an open forum like this, so I really can't answer one way or another."This is less than 3 months after he sent a letter to congress saying that they were investigating Emails that were linked to Hilary Clinton. Two weeks before the election. Now I know that wasn't a comment in an open forum but still . . .
The reason for not making comments on ongoing investigations is that it removes all questions of motive and timing in disclosing information. The letter to a political opponent of a presidential candidate looks like you are trying to evade the norm. Plus, once you do it once, you are stuck explaining why different rules apply when these questions arise in other contexts. And it's hard to justify deciding when to comment or not on ongoing investigations when there is a partisan element.
I can recognize that there is a key difference of kind here (letter to congress versus open comment) but these are awfully fine distinctions. Would he provide a letter if Democrats senators asked for one? These perceptions are now putting pressure on what is a very important social norm of a key investigative body. I hope that they can figure out a way forward that brings back the old norms, which had some really good reasons to exist.