When would it have been alright for the FBI to release the update? If they waited until after the election, the other side would accuse them of covering it up—appropriately so.
And as an aside, it was an update to the effect of "we don't actually know anything new, but we found something that might hold something new." And, as we know now, didn't actually contain anything new. There's a reason they call it the October surprise.
http://www.wsj.com/articles/u-s-blames-russia-for-recent-hac...
At an absolute minimum they should have waited until they knew there was some data both relevant to the investigation and not previously known to the FBI. They should probably also have made sure the evidence implied wrongdoing, out of concern of influencing the election.
This is exactly what Comey was told by the Justice Department, his subordinates, and pretty much everyone else.
http://www.nytimes.com/2016/10/30/us/politics/comey-clinton-...
The FBI's and Justice Department's long-standing rules on the issue are: 1) Never discuss ongoing investigations; and 2) take great care to never say anything that will at all affect an election.
So while your question is an interesting one, the FBI has two clear, long-standing answers and rules, and they violated them.
As a rule the FBI simply does not announce they're investigating someone because it can damage the investigation, and because it could unfairly tarnish someone who has done nothing wrong.
The encroachment of political calculus into the Justice Department is not a good thing. Let Democrats or Republicans howl about whatever, the DoJ and other law enforcement and intelligence agencies need to do their job the right way, regardless of partisan complaining.
That has no significance when weighed against the risk undermining democracy and effectively choosing the next President, which is what happened.