So, TerraPass just got done with a customer survey, with several thousand responses. Uncovered was the shocking news that people are not, in fact, using offsets as an excuse to indulge in other bad behaviors. (Here’s a one-page PDF summarizing results.) In fact, just as you’d expect, people who care enough to pay for offsets also care enough to reduce their emissions in other ways.
As Adam says, this counts as pretty good evidence against the "indulgences" theory of offsets, particularly in light of the fact that there’s no evidence for it.
Of course, it’s possible that TerraPass customers are lying, and they really are buying SUVs because they bought offsets, but absent any reason to think so, maybe we can drop this odd, moralistic trope?