I learned a long time ago that nearly everything is subjective, and in the case of disciplinary councils, I believe rightly so, the public only hears the individual's side of the situation. We are not supposed to hear the other side (the institutional side) - and I think that is appropriate. Those who participate in these councils pledge to not talk about the details, and that is an important and valuable thing with which I agree totally. What it does, however, is allow those who are displeased with the decision to be the only public voice - and memories and perspectives while under stress are incredibly tricky things.
Reaching a "proper" decision becomes especially difficult whenever mental or emotional issues are present, since it's almost impossible to trust completely that the person who has such issues will be able to control them in a way that allows them to avoid repeat offenses. Add to that the automatic confidence of many that they won't sin again, even if there isn't evidence of that ability, and disfellowshipment becomes a reasonable "probationary" measure for many instances - a chance to delay a more extreme action (in either direction) and allow the person to prove themselves (one way or the other).
In my experience, disfellowshipment often is the most merciful decision possible - especially when the offense truly is serious and remorse seems real. Just like a time out or grounding or removal of privilege for a child can impress on him the seriousness of something done, without having to resort to extreme measures, disfellowshipment can do the same in tricky or complex situations where the council wants to believe the person but can't have full confidence yet.
In the end, I try not to judge anyone involved in a disciplinary council (in any position in that council), since I know I never will have the full story - sometimes even when I'm involved in it personally.
Having said that, and knowing how often excommunications result in long-term or permanent separation from the Church, one of the most amazing spiritual experiences of my life was a council where someone was cleared to be re-baptized after being excommunicated. I will NEVER forget that glorious experience. When conducted as intended, for the truly penitent, these councils are an amazing thing.