I've actually had the opposite experience as you. Having my debit card keeps an easy running tally which is difficult to do unless you only leave with small sums of cash in your pockets. However, when you only have a small amount of cash you run into the "just short" effect, where all you need is a few dollars more to get what you want and you can afford it but can't get it. To me, the potential opportunity cost is just not worth it.
I have found it difficult to track cash spending. I picked up using GNUCash this year, and that helped a bit. I at least can capture most cash spending to see if I'm within budget in different categories. But every few week's I still need to make a 'correction' transaction for ~$10, which is annoying.
Call me naive but, as a dev, I don't see why this would be any better than using JS. The group of people that block JS is likely to do the same for this and, as mentioned by others, common sources of such mucking are blocked by a good ad blocker.
Then, there is the whole, "how could it be integrated into an existing site with minimal fuss" issue. With JS you can specify targets and the like for actions and observations, the only comparable thing would be to offer sass / less integration so that it works with clients that disable or block JS, which is arguably much more difficult.
While it is definitely clever, I just don't see a practical use for it. It would really only benefit those willing to put the work into using it and only work so long as their logging URL is available and not blocked. I just don't see the real value.
