Hrm, I think you might be ignoring the cost of actually doing the calculations, unless I'm missing something. The value of simplifying assumptions comes from how much easier it makes a situation to model. I guess the question would be, is the effort saved in modeling this thing with an approximation rather than exact figures worth the risks of modeling this thing with an approximation rather than exact figures? Especially if you have to do many models like this, or model a lot of other factors as well. Such as trying to sort out what are the best ways to spend your time overall, including possibly meteorite preparations.
What is the probability that my apartment will be struck by a meteorite tomorrow? Based on the information I have, I might say something like 10-18. Now suppose I wanted to approximate that probability with a different number. Which is a better approximation: 0 or 1/2?
The answer depends on what we mean by "better," and this is a situation where epistemic (truthseeking) and instrumental (useful) rationality will disagree.
As an epistemic rationalist, I would say that 1/2 is a better approximation than 0, because the Kullback-Leibler Divergence is (about) 1 bit for the former, and infinity for the latter. This means that my expected Bayes Score drops by one bit if I use 1/2 instead of 10-18, but it drops to minus infinity if I use 0, and any probability conditional on a meteorite striking my apartment would be undefined; if a meteorite did indeed strike, I would instantly fall to the lowest layer of Bayesian hell. This is too horrible a fate to imagine, so I would have to go with a probability of 1/2.
As an instrumental rationalist, I would say that 0 is a better approximation than 1/2. Even if a meteorite does strike my apartment, I will suffer only a finite amount of harm. If I'm still alive, I won't lose all of my powers as a predictor, even if I assigned a probability of 0; I will simply rationalize some other explanation for the destruction of my apartment. Assigning a probability of 1/2 would force me to actually plan for the meteorite strike, perhaps by moving all of my stuff out of the apartment. This is a totally unreasonable price to pay, so I would have to go with a probability of 0.
I hope this can be a simple and uncontroversial example of the difference between epistemic and instrumental rationality. While the normative theory of probabilities is the same for any rationalist, the sorts of approximations a bounded rationalist would prefer can differ very much.