> I would argue that financial math by definition needs to be accurate to the penny. Where is "pretty close" financial calculations considered acceptable?

There is a difference between analysis and accounting. There are many financial models (e.g. Black-Scholes-Merton option pricing) that are analytic in nature and use transcendental functions, so the idea of getting an exact, arbitrary-precision answer is hopeless. Using a decimal type for this kind of computation would be an exercise in slowing down processing by a few orders of magnitude.