Let's not forget that Hillary wasn't some innocent in this, she was under investigation and to discredit that as a catalyst or a point of contention that would be leveraged is naive. In many ways, her actions and the storm around them brought heightened exposure to the sensitivities of the topic and made for a great way to push and divide the country even more. She's not a victim and in fact I would argue she too was part of the problem.
You're right that Clinton was already under investigation for Benghazi, but that's a sign that the correct domestic legal process was being followed. The FBI gathered evidence and she testified before congressional hearings. That's how things are supposed to go.
Moreover, Hillary doesn't matter any more, in the same way that McCain doesn't (hence the OP referring to Obama only). She is not President, nor is she playing any sizable role in public life. Deflecting the substantive and meaningful criticism of the sitting President with "but Hillary" is at best distracting, and at worst a concerted effort to avoid Trump having to answer very legitimate questions.
As for Deflecting it's the driving force behind, he shouldn't have won, she should have, so no unlike the others it is relevant.
