We should avoid the irony of division over the meaning of this text; however, it is most faithful to the text to see it as sarcasm.
Paul is Rebuking the Corinthians
It is not insignificant for demonstrating their position that Ciampa and Rosner note that Paul is rebuking the Corinthians. I would like to hear how the context plays into Dr. MacArthur's interpretation. This is a rather divergent aside if Paul is here laying out a doctrine of wheat and tares; one would expect that in a passage intended to provide comfort to a church plagued by heresies, not as a blip in the middle of a rebuke.
Remember also that this is not just the narrow context of this passage; the main thrust of the whole letter is to reprimand a wayward church. It is hardly expected for Paul to start extolling the good aspects of division right in the middle.
Division in the Corinthian Letters
This argument is strengthened when you look at Paul's other mentions of divisions in this letter, and to some extent in the next. This is the verse major topic of the letter—by verse 10, Paul is into it. And lest we think he gives it up quickly, it turns out in chapter 3 verse 1 that he is still addressing that issue. In fact, he continues on that note for all of chapter 3 and most of 4, moving on to speak about spiritual pride in the last verses of 4 and on into 5. Then turn over to six—low and behold, he is still speaking about divisions! From then on, as he moves to deal with a variety of theological questions, including disorder in the worship service, it should be bountifully clear that he is trying to unify them by stopping their divisions of these questions. Not how in his discussion of food offered to idols (chapter 8), he is not as concerned about who is right as that both sides be considerate of one another. And it is very significant that he interprets himself in 13 to give a long discourse on love.
Do you see the significance of this? Discord is the theme of the whole letter. When Paul says therefore late in the letter, "I've heard there is discord among you! I'm not sure whether to believe it!" we ought to recognize that the sarcasm is scalding.
Paul's Partial Disbelief
Paul does not speak of his partial belief because he thinks it to be theologically improbable that they are not unified. The sense is not, "theologically, I thought about it, and I can see both that it is important for the church to be unified, but also that disunity has its place." Rather he means, "I want to give you the benefit of the doubt; however, it's clear that you actually are disunified." In fact, I am inclined to interpret it as even harsher than that—"What? The Corinthians disunified? No way!" Yes way, guys, that's just what I've been talking about through the whole letter. Who's surprised to find another way in which you are disunified?
The Corinthian Letters on Church Discipline
In chapter 5, Paul makes a clear statement that a man living in unrepentant sin ought to be cast out of the church. As such, it hardly makes sense for Paul to see divisions among the Corinthian church as a good thing. Here and elsewhere in the new Testament, those who are giving false teaching or living in opposition to right teaching are to be cast out of the church.
Remember, in this verse Paul is not pontificating about the state of the church as a whole, but is speaking to the church in a specific city. He does not say, "divisions among us," i.e., the whole church, but "among you". Unlike modern American slang in which "you" is a sort of catch-all pronoun, Paul is very deliberate about his use of "you" and "us" in his letters.
As such, from the vantage point of this letter, heretics ought not to be considered part of the church. The church out to be unified; if there are those who would disturb, let them remain on the outside.
Sarcasm/Irony in the Corinthian Letters
There can be a propensity when reading Scripture to forget that it does contain many sarcastic or ironic passages; God's caustic denouncements of the idols in the Old Testament are often dripping with it. Isaiah 44:13 is a good example:
The carpenter measures with a line and makes an outline with a marker; he roughs it out with chisels and marks it with compasses. He shapes it in the form of man, of man in all his glory, that it may dwell in a shrine. (NIV)
More to the point, Paul is known to use sarcasm and irony, particularly in his letters to the the Corinthians. In fact, in this context the irony starts in verse 17: "But now, when I mention the next issue, I cannot praise you." Hmm. First Corinthians is not exactly a letter of resounding praise before the second half of chapter 11 either.
In chapters 1-3, there is the irony of the "foolishness" of the wisdom of God and the foolishness of the "wisdom" of the world.
Chapter 4 verse 8 is ironic, if not sarcastic:
Already you have all you want! Already you have become rich! You have become kings—and that without us! How I wish that you really had become kings so that we might be kings with you! (NIV)
Chapter 5 verses 12 and 13 should also be read in a mocking tone:
"Everything is permissible for me"—but not everything is beneficial. "Everything is permissible for me"—but I will not be mastered by anything. "Food for the stomach and the stomach for food"—but God will destroy them both. (NIV)
Like Jesus often did to the Pharisees, in these verses Paul does not directly contradict the Corinthians. Rather he sideswipes them and leaves it to them to them to recognize just how far they are in error. 5:12-13 is not coming in the middle of his discussion on food, but in the middle of his discussion on sexuality. Paul is the adamant opponent of this antinomianism (Romans 6)! Translations are right to put quotes around those phrases; Paul is not affirming them. The same is true of 10:23 and possibly 12:31. In fact, throughout the whole letter Paul seems to be in dialogue with a letter which he received from them (e.g. 7:1).
In 8:1-2 he is also speaking tongue-in-cheek. Indeed, I could go on and make an examination of 2 Corinthians in a similar manner, but I think the point is sufficiently clear. The letter is laced with cases where Paul's meaning is not as simple as the words on the page.
The best reading of this verse is to understand it as Paul mocking their spiritual pride.