At the time Paul wrote this, "all scripture is inspired of God, and is profitable" referred to the Old Testament. Paul probably did not know that what he was writing would become Scripture.
At its worst, saying that the law—or any portion of the Bible—is "done away" could be spiritually suicidal. At the very least, it will hinder growth because a person will not be thoroughly furnished to all good works. It is similar in principle to a student attending school who ignores certain selected sections of the textbook on the basis of his own perception of what he needs.
I recall from my own school days expressing the opinion that I could not see why we had to study ancient Greek, Egyptian, or Roman history. I could not see what good I would ever get out of such courses, but others, much older and wiser, insisted that the history we were taught included teaching in these areas. My narrow point of view was that of an immature kid who did not understand what is required to produce a well-rounded citizen of the United States of America.
In a similar manner, but with far greater accuracy and consequences, there is nothing extraneous in God's Word. We are to live by every Word of God (Matthew 4:4). If God is all-wise and all-powerful, if everything that He does is in love, and if He is working out a purpose that is in our best interest (that we might live forever with Him), why would God even give a body of laws—which Jesus said would never pass away until all is fulfilled (Matthew 5:18), and which Paul wrote is spiritual, holy, just, and good (Romans 7:12)—if God did not intend that its letter and/or spirit should be used for all time? God does nothing without meaning, so the law is included in Scripture for the sake of Christians.