It's hard to say - I've spent a lot of time working in a force-paired environment and also in a pairing-optional environment. The highest quality code I've seen is not in the paired environment. That probably has more to do with the caliber and discipline of the individual developers. Sometimes you'll get your money's worth from pairing, especially where some of the coders are not top-tier. But if all the coders are experienced, well-disciplined coders, you're just wasting your money if they're pairing all the time.
One type of experience that I've had multiple times that has a huge impact on my coding discipline and the quality of product I produce is this: carrying a pager. When I have to support a system for which I code, it changes how I code. That is, I code in such a way as to keep that pager from going off. The result is a better quality product, and typically better code quality, too. The coders I've seen that have never carried a pager produce code that's more fragile. It's not something that they can even understand and improve upon until they've been on support.
