There are two kinds of people who cannot see the error of unionism and syncretism. The first are those who allow what we call today "Contemporary Worship" (CoWo). The second are those who are theological liberals. If you look at those arguing in defense of Pastor Morris' sin on the various blogs they are the same usual suspects who are defending CoWo and also those who support the various hallmarks of liberalism such as open communion, women's ordination, and the like.
Before we get into the details on why each cannot even see the error, let's begin with a definition of unionism and syncretism.
Unionism is "taking part in the services and sacramental rites of heterodox congregations or of congregations of mixed confession." (Constitution, Article VI., Paragraph 2. b., LCMS 2010 Handbook, p.15). The emphasis is on "services and sacramental rites." That means that a pastor saying a prayer at a Fourth of July rally to start the annual community parade is just fine but saying a prayer at Fourth of July community worship service is forbidden. We will say more about why it is forbidden below. Unionism is when a pastor does this with heterodox Christians (those who mix false teaching in with true teaching such as Roman Catholics, Baptists, Methodists, etc.). Syncretism refers to the same only with pagans (Muslims, Jews, Hindus, etc.).
When I listen to the CoWo and liberal crowd talk about the motivation for their unionistic and syncretistic urge I hear them give various reasons including a need to demonstrate unity with other Christians (unionism) and other religious types (syncretism), to be loving, and to reach out to them so that we can make progress toward unity.
Each of these reasons is lacking. Unionism and syncretism do not demonstrate unity. They do the exact opposite. Unity is based on agreement in God's word. Unionism and syncretism gloss over true unity and demonstrates diversity, not unity. Likewise it is not a loving thing to do. It shows a lack of care for the heterodox and the pagans. The loving thing to do is to teach them the right doctrine. Also, to make progress toward unity with the heterodox and pagans one does not need to confuse the matter by joining them in the worship of a co-mingled god. There are countless other ways to do this.
All three of these things can be done without entering into services and sacramental rites of the heterodox and pagans. I can demonstrate the unity I do have with them by talking over coffee rather than by joining in the false spiritual intercourse of services and sacramental rites. I can teach them and make progress toward unity by leading seminars, meeting with them in their homes, inviting them to my church, etc.
So why is it that the CoWo crowd can't see these things? CoWo is caused by a desire to get more from worship than what the 2,000 years of forms and orders that traditional, liturgical worship and preaching provide. In worship the CoWo crowd wants the emotion and feeling that comes with pop culture. They want this because they see worship as our work for God rather than His Divine Service to us of the forgiveness in Holy Baptism, Holy Absolution (confession/absolution and preaching) and Holy Communion. Emotion is fine in the Divine Service. I know of nothing more emotional and intimate than receiving His body and blood. You cannot get any closer to God than what happens in Holy Communion. For that matter the same can be said of preaching and baptism. In baptism the Holy Spirit joins himself to you and in preaching we are feeding on the Word of Christ, but emotions are not the main thing in worship.
When you say "I am not getting enough out of the traditional liturgy. I need to add a pop culture vibe to it. I am not really worshipping when I am stuck in a traditional form." you are inherently saying that you never did understand the means of grace that the traditional liturgy revolves around. The means of grace are what it is all about and the traditional liturgy provides a time tested format that keeps the means front and center and not me, my emotions, my cultural taste in music and my felt need to worship God. There is room for pop Christian music in the traditional liturgy. The LSB has some time-tested and doctrinally pure pop Christian music in it. The problem with a totally CoWo service is the emphasis mentioned above and the fact that most CoWo music is written by heterodox Christians who do not know the Biblical doctrine of the means of grace and who think that worship is all about them coming to God and getting lathered up in an emotional experience, not to mention the subtle works-righteousness in most of CoWo music.
How about the liberals? Why can't they see the errors of unionism and syncretism? It is because at the heart of their false spirituality lies a doubt in the inerrancy of God's word. They allow current social mores to determine their practice rather than the word of God. Where God's word says something that is difficult, like "a woman is to keep silent in the church," "one is not to commune without the ability to discern the body and blood of Christ," or "mark and avoid them" (false teachers), they choose the easy path offered by the culture rather than the difficult path taught in God's word. If you do not commit to the fact that God's word is true in every single phrase, word, jot and tittle, (inerrancy) then you will cave to the culture and which is nothng mroe than starting down a long "slip and slide" that eventually leaves you with no truth at all.
Faith in God brings one into the most intimate union with God. Holy communion makes us the bride of Christ. As Paul and the prophets teach, to take Christ into the heterodox and pagan lair is to prostitute Him. There was a whole generation of us Lutherans, maybe even a generation and a half of us, that took the sacrament of Holy Communion for granted. That is what led to so many of us dabbling in CoWo (myself included). We did not realize that we had all that we needed in the historical, traditional liturgy. We went searching for more. Liturgical renewal has taught us the proper piety of the Lord's Supper. There is no more greater union with God than that of His Supper, His Baptism and the eating of His Word in preaching and absolution.
It is going to be a long road back to this in the LCMS. Some will just never swallow their pride and accept it. They are the 15% mentioned in the Koinonia project who will need to be left behind in their false evangelicalism, liberalism, millennialism, and spiritual narcissism. If you will, keep your eyes open to the false spirituality of unionism and syncretism and help our synod president get us back to the truth and purity of the Gospel.