Let me picture this. You are standing at the checkout of a grocery store with a big shopping basket, and you find out that you are over budget for today ... Not sure if I would like this functionality.
How is that different from not having the money to spend in the first place, though?
Even people with very little money use credit occasionally, and I can definitely see the benefit of strong-arming a budget so you don't spend money you don't have.
The typical reaction to this scenario is to put back what you have at the grocery store and leave (or put back enough so that you _can_ afford it).
(It could be argued that you don't need a feature like this at all if you have X level of self-control, but many people would benefit from taking any requirement of self-control out of the equation.)
Let me picture this. You are standing at the checkout of a grocery store with a big shopping basket, and you find out that you are over budget for today ... Not sure if I would like this functionality.