We all make mistakes -- that's why pencils have erasers and renters have insurance. Not all mistakes, however, are equal. If we mess up a math test, we can erase and start over; but if we burn the neighbor's house down, we can't "un-burn" it by labeling it a mistake. Instead, we have to offset our damage by paying to rebuild it.
Offsetting isn't a complete and total "fix" to the climate mess, and it isn't intended to be. Poorly implemented, it may even become the ploy the pope worries about. But done right, it's an incredibly effective tool in a very large toolbox, yet it will only deliver on a large scale if it's embedded in a functional, well-regulated, and global climate-change response.
The voluntary carbon markets have served as the testing ground for compliance programs all over the world, even moving forward when efforts to implement mandatory cap-and-trade programs stalled. This has led to the voluntary markets having an influence that extends well beyond the nearly 1 billion offsets transacted over the last decade.