Ok, some more thoughts on church unity.
I'm not familiar enough with other general strands of protestantism to know how prevalent (or not) this is, but there seems to be a strong tendency in Presbyterianism to divide. I point you to a diagram of Presbyterian splits and merges just in the United States.
Does anyone really think that diagram looks like unity? Looks more like spaghetti to me.
I won't claim to know or understand what all the reasons were for the splits and recombinations. I know the PCA and OPC split off from their respective branches over doctrine. The Civil War was also a factor in that diagram. You can see that we also carried over some differences from homelands. At any rate, this splitting results in what I have seen called "micro-denominations." I haven't seen a definition for that term (though doing a Google search on it was interesting) but it certainly implies a small denomination that holds certain distinctives very dear. I would say there are a few in NAPARC.
Of course, this is something of a matter of perspective. Any Christian denomination in, say, the Middle East would likely be a micro-denomination. And let me be clear, I believe it is one thing for a family to decide where to go to church and a very different thing for a church to decide what denomination they will be a part of. I'm talking about the latter decision.
My main point is to wonder why a micro-denomination (at least of Presbyterians) would exist in the United States. We have Presbyterian denominations of decent size from liberal to conservative and on many other dimensions. Besides those in NAPARC, there is the mainline PCUSA, the Cumberland Presbyterians, and the EPC. A church has lots of choices already - why pick (or start) a tiny one? I suppose I would particularly wonder that in light of the Church being called to unity. It also seems a huge practical expense to maintain a denominational structure when you are tiny. Do you have a educational materials office or just use another denomination's materials? How do you have presbytery meetings when pastors are spread over hundreds of miles across multiple states? Do you develop your own set of polices from scratch?
I think a denomination's very existence says: "we are correct and everyone else is wrong on this slate of important issues/doctrines/practices." And those micro-denominations hold some thing or things very dear that simply do not overlap with other, larger bodies. If a church is convinced that certain things are non-negotiable, that there is no debate to be had on them, then I'll admit, I don't see much choice for them. If you are convinced from scripture that you are to hold worship services on Sunday morning from day break to noon and everyone is supposed to stand the whole time (see Nehemiah 8), then I can see a church having a problem with a denomination that doesn't support that requirement. But then, surely you have to wonder if you're the tiny minority, are you the faithful Remnant pursuing holiness before God, or are you wrong and sinfully hurting the unity of the body?
That's not a bad question to ask about any denomination as long as such entities exist. The PCA, or all of NAPARC put together wouldn't do wrong to ask which one we are. Perhaps I feel insulated from the question because I perceive the PCA as big (and compared to some they are). However, compared to 1 billion Catholics, 300,000 PCA members is rather tiny.