/A/ reason not to spend much time thinking about the I-am-undetectably-insane scenario is as you describe; however, it's not the /only/ reason not to spend much time thinking about it.
I often have trouble explaining myself, and need multiple descriptions of an idea to get a point across, so allow me to try again:
There is roughly a 30 out of 1,000,000 chance that I will die in the next 24 hours. Over a week, simplifying a bit, that's roughly 200 out of 1,000,000 odds of me dying. If I were to buy a 1-in-a-million lottery ticket a week, then, by one rule of thumb, I should be spending 200 times as much of my attention on my forthcoming demise than I should on buying that ticket and imagining what to do with the winnings.
In parallel, if I am to compare two independent scenarios, the at-least-one-in-ten-billion odds that I'm hallucinating all this, and the darned-near-zero odds of a Pascal's Mugging attempt, then I should be spending proportionately that much more time dealing with the Matrix scenario than that the Pascal's Mugging attempt is true; which works out to darned-near-zero seconds spent bothering with the Mugging, no matter how much or how little time I spend contemplating the Matrix.
(There are, of course, alternative viewpoints which may make it worth spending more time on the low-probability scenarios in each case; for example, buying a lottery ticket can be viewed as one of the few low-cost ways to funnel money from most of your parallel-universe selves so that a certain few of your parallel-universe selves have enough resources to work on certain projects that are otherwise infeasibly expensive. But these alternatives require careful consideration and construction, at least enough to be able to have enough logical weight behind them to counter the standard rule-of-thumb I'm trying to propose here.)
In parallel, if I am to compare two independent scenarios, the at-least-one-in-ten-billion odds that I'm hallucinating all this, and the darned-near-zero odds of a Pascal's Mugging attempt, then I should be spending proportionately that much more time dealing with the Matrix scenario than that the Pascal's Mugging attempt is true
That still sounds wrong. You appear to be deciding on what to precompute for purely by probability, without considering that some possible futures will give you the chance to shift more utility around.
If I don't know anything ab...
Summary: the problem with Pascal's Mugging arguments is that, intuitively, some probabilities are just too small to care about. There might be a principled reason for ignoring some probabilities, namely that they violate an implicit assumption behind expected utility theory. This suggests a possible approach for formally defining a "probability small enough to ignore", though there's still a bit of arbitrariness in it.