Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven. (Mt. 18:18)
The modern and post-modern deconfessional era has resulted in the formation of many many denominations and independent churches, not all of them biblical. One major problem that arises out of this is the issue of church discipline and excommunication. In a place with many churches not in communion with each other, someone who is excommunicated can simply hop over to the next church down the street and be received with open arms. Church discipline and excommunication, in this sense, has become impractical. What's the point of going through all that effort only to see that member leaving, complete with broken relationships and less incoming tithe?
The fundamental issue to be dealt with is the purpose of discipline and excommunication. Is it merely for the purpose of depriving a person of Christian fellowship and of partaking the sacraments as it were (i.e. the Anabaptist idea of shunning)? If so, then certainly the plurality of churches will render that goal impossible to reach. If however, we see discipline and excommunication as judicial acts of the Church, which if truly in line with Scripture conveys the actual sentence of Christ in this present age, then our perspective should change.
Matthew 18:18 and the keys of the kingdom speak about the authority Christ gave to the Apostles and through them the Ministers of the Gospel. All of them have the keys of the kingdom. The key of loosing is the proclamation of the Gospel opening the door to salvation. But what is the key of binding? It is the process of church discipline and excommunication. To those who are persistently recalcitrant and unrepentant in their sins, God in Christ has given the key of binding through which these people are put out of the kingdom of God. As a judicial act of the Church, when such is done in line with God's precepts, it is as if Christ has came and pronounced the sentence Himself. What is bound on earth WILL be bound in heaven. Christ has vested His ministers with this authority that they yield on His behalf, and thus excommunication has actual power, not just made up of ineffectual empty sentences by the courts of the church.
What this means is that even though church discipline and excommunication may seem ineffectual, since a member can just go to the next church down the street, a biblically valid excommunication is the decree by the Church upon which God looks with disfavor upon the one under its sentence. The weapons of the Church are spiritual, and on this issue, it is most certainly spiritual. The Church has no physical or governmental power (or at least it shouldn't have), but it is certainly far better to be punished by Man than by God. Someone under legitimate excommunication has God as his enemy, and that is NOT something anyone should ever want. If the person is elect, God will severely chastise him until he repents and submits to the discipline of the Church.
So yes, in an era where a person can just walk down the street to the next church and ignore whatever his previous church has said, he seems fine practically. But things in the spiritual realm are far different, and that is what matters.
For those who actually believe and practice church discipline, which is a mark of the true church, this should inform our practice in regards to accepting professing believers from other churches. It doesn't really matter what church they are from, if the person is being subjected to church discipline or even excommunicated, we have to find out why. If the reason is invalid (e.g. promoting Reformed theology in an Arminian church), one can disregard the prior discipline as being one contrary to Scripture. If however the reason is valid (i.e. wife-beating, illegitimate divorce), the sentence of excommunication must be upheld even if the sentence came from a Pentecostal church.
It was the Reformation that came up with the three marks of the true church, one of which was proper administration of church discipline. In this era of degenerate Christendom, we should most certainly be concerned over the practical ineffectiveness of church discipline and excommunication, yet continue to do so regardless of its supposed practicality. Ultimately, God is the One we serve. At the very least, proper execution of church discipline would bar the person from going back to true churches where the mark of discipline is practiced.