While I understand that logic, I also find it a bit distasteful. Law enforcement shouldn't tolerate crimes being committed on its watch any longer than necessary.
Of course, the exact meaning of "necessary" is the problem. We can probably all agree that letting a serial murderer kill more people in order to have more evidence against him would be an absolute abomination. But the situation is not that clear with white-collar crime, especially the sort of crime where the harm is not immediately obvious.
Maybe the fact that the FBI could wait that long before pouncing indicates that this sort of betting is, in fact, a victimless crime.