Rebuke is one of the 613 commandments. And where you can see the serious of it is in the events surrounding the concubine in Giva. In short for those who are not familiar with those events let me recount the major points. One fellow had a concubine. [That in itself is subject to a debate. Some hold that is simply what you would call a girl friend, i.e. sex with no marriage. Others hold there is kidushin, but not nisuin/hupa.] He visited her parents home with her, and then wanted to get back up north where his home was. His concubine wanted to stay and bit longer, but he insisted they get back home. On the way they passed through the area of the tribe of Benjamin in a town called Giva. There were some ruffians there that attacked them at night and killed her.
But worse than that--no one else in that town objected. So all Israel made war against the tribe of Benjamin. And they even asked God directly by means of the Urim and Tumim and God approved of that war. No because of the murder, but because no one besides the actual criminal objected.
Another place to see this is in the events of Kamtza and Bar Kamtza by which the Second Temple was destroyed. The reason was not so much the actual crime, but rather that the sages at the time did not object.
So when we see the Gra signed the well known letter of excommunication (and Rav Shach also agreed with the Gra) and yet their objections have been ignored, I feel that I ought to say something myself and encourage others. I mean to say that when asked I have stated my view that the Gra was right. But now I am seeing that it is not enough to for me to do so. Others also ought to go along with the Gra.]
I also think that besides the fact that a herem/excommunication is a valid halachic category; there is also the issue of why that herem was issued in the first place (i.e. idolatry). And time has shown that the Gra was right.
[The herem did not apply to Rav Nahman of Breslov as it was specific. So learning Rav Nahman's books is perfectly fine and commendable.]
