St.John of Kronstadt was a gifted preacher, and a many who manifested great sanctity - he was a parish Priest known also for the working of miracles and other extraordinary gifts of grace.
Often times his parish would be so packed when he served, there was no possibility of being able to confess everyone; many prodigals came back to faith because of his work.
So, as a manifestation of leniency, St.John had a form of "common confession", but it was not what one typically see's today (and which is why I think the citing of St.John as a proponent of what we now see as "common confession" is mistaken); it involved people literally shouting out their sins, literally confessing in public (and before God) in the Church, while St.John stood as a witness, and then offered the absolution.
As for the contemporary practice, my take on it is this: I am not going to say it is entirely worthless, or somehow intrinsically "invalid." The problem is really a question of prudence and pastoral responsibility.
In case no one has noticed, "confession" has never been a very popular thing. It hurts, it's uncomfortable, to let anyone into the ugliness we sadly find in our hearts. It's the same discomfort and shame our first parents found in their nakedness - to confess one's sins in front of another, is to show our nakedness, our shame. It is humilitating - though let's remember the root of that, word; humility.
If circumstances warrant it (such as some kind of emergency), then perhaps such an act of "general absolution" is appropriate. There is an inherent danger in such acts however, and it is one which I only think becomes an "acceptable risk" in such dire situations.
That risk, is that such practices of "general absolution" can become a "back door" for people, and foster the conceit that there is an "easier way" to get right with God. The unfortunate truth is that it is "easy", particularly for sickly souls (and I definately include myself in that number), to convince themselves that they have "repented" without bearing the fruits of repentence. While some may gasp at my saying this (ex. "how can you demean the value of someone repenting privately to God?" - which I am not, btw.), consider that our vile sins as Christians were done all the while we were allegedly those who "believe in God" and "believe God see's us and is present wherever we go". Yet, did such a God-consciousness prevent us from sinning? Did the knowledge of God's presence and omniscience prevent our fornications, or our thefts? While one could argue that if it were firm enough, it certainly would have...the fact is, in the case of such poor sinners, it did not.
Given this, there is always the possibility that when "repentence" remains a completely private affair, that it can be comprimised; we may not truly appreciate the seriousness of what we have done, and not come away from the experience of such a private confession with a renewed humility.
IMHO the "general confession" that is being discussed here with it's "general absolutions" is fundamentally an act of private repentence/confession, but simply with the add on of a Priestly absolution, taking place in a congregational setting.
Whenever we sin, we manifest some basic problem with our faith; this is particularly so if we sin grievously. The reality of Christ's presence, and a sufficiently serious appreciation of this, is facilitated by the visible presence of His Priest, and when he is included in that act of contrition - in particular, included in the awful reality and basic details of how we have failed.
Just as the act of martyrdom which flows from the open confession of Christ Jesus proves the sincerity of that which could have remained in some wise debatable otherwise (that one loves the Master more than anything else, including his own life), confession of sins before the Priest better demonstrates and assures the sincerity of the penitent, given that it is a contrition which has come at a price.
This is also beside the fact, that there is a great deal in the way of spiritual counsel and personalized correction that one will not receive, apart from traditional sacramental confession.
Another aspect of this, which I would hope Priests themselves would keep in mind, is the personal responsibility (before God and His Church) which the Priest has as guardian and administrator of the Holy Mysteries (which traditional, personal confession has been a tool in better guaranteeing.) A Priest who shirks his responsibility in this charge, or carelessly allows people to persist in some kind of delusion and only add to their sins by the sacreligious reception of sacraments, will be held responsible for this by God. This is a terrible thing, and is why even under the best of circumstances I'd have no desire to be a Priest; could it not be called great folly, for a Priest to do that which is liable to harm his flock, particularly it's weaker members?