Well, Carl Trueman doesn't answer the question for Calvin PCA specifically, but he does give a good answer as to why the great Protestant confessions are so important to evangelical church life today. From the new book, Risking the Truth: Handling Error in the Church (edited by Martin Downes), Carl Trueman answers the question, "Why have evangelicals reduced the great Protestant confessions down to minimal statements?" Here's part of his answer:
Because evangelicalism, as a transdenominational, parachurch movement, needs to sideline great swathes of the faith in order to hold the alliance together. That is not a bad thing in itself.... Such parachurch alliances are important in presenting a popular front for the gospel in the current climate.
Popular front evangelicalism only becomes a problem when, with its minimal doctrinal basis, it comes to be normative for how we actually understand Christianity and thus to impact how we understand the church. Then we find ourselves in a situation where tail wags dog, so to speak, where the identity of the church is shaped not by her own confession but by the exigencies of the evangelical world, where key theological issues such as divine sovereignty, baptism, and the Lord's Supper are marginalized. Wherever we come down on these issues, Scripture does teach about them; and we have no right to make them merely negotiable matters of indifference in the church. At the ecclesiastical level, I would rather do business with a convinced Arminian or Baptist who knows that the Bible's teaching on the pertinent issues are important, than with someone who thinks it is all a bit unclear and not that vital anyway.
The long-term impact of abandoning the historic confessions and catechisms is wide-ranging. You stand to lose much historical identity and sense of continuity with the past. With no catechisms and confessions of any depth, you have few resources left in the face of a rising tide of theological illiteracy which leaves the way open for all manner of weird and wonderful stuff to fill the resulting vacuum. You can end up simply replacing them with doctrinal statements which, through their very minimal nature are inherently unstable. And you might find you have a theology which is unsatisfying and ultimately of little use in providing a base from which to address many of the great issues of life.
None of this is to invest historic confessions and catechisms with the authority proper to Scripture alone; but it is to point to them as serious ecclesiastical and historical attempts to wrestle with the great themes of Scripture. If you wish to abandon them, you are free to do so; but unless you can find something which does the job equally well, in just as comprehensive and catholic a fashion, you might want to think twice before you throw them away.
The book from which this excerpt was taken looks to be an important contribution in dealing with various kinds of error within the church (I have an excerpt of the book, but my full copy is on order). As a minister of the gospel I am constantly confronted with the personal need to guard against theological error. As James so clearly reminds all who teach, our judgment will be stricter (James 3:1). So, as I approach the pulpit each Sunday, my fear and trembling is tempered by the knowledge of God's grace, the understanding that His Spirit is at work despite my lame tongue, and also by the fact that I have a faithful and reliable guide in understanding God's Word, the Westminster Confession of Faith. Indeed, as Trueman affirms, the WCF does not hold authority on par with Scripture, but it has been received and approved by Christ's church for generations as a reliable guide to the truths of God's Word. And if you are prone to stray into the "weird and wonderful" (Trueman's words, again), nothing can be more helpful than the vast wisdom of Christ's church passed on through the ages in the form of the great Protestant confessions.