Suppose that a student submitted an essay to a professor that is suspiciously similar to an essay submitted to the professor a previous term. Hypothesis A might be the second student copied from the first and hypothesis B would be that they both copied independently from a third source. How would the professor go about determining which hypothesis is more likely?
Hypothesis A is somewhat more parsimonious than B in that there is no need to posit the existence of a third source, so Okkam's Razor might favor A. On the other hand, it is easy to see how the effect of parsimony would be quickly overwhelmed by evidential factors. For example, if investigation established that the two students were friends, that would incline the professor towards A much more than mere parsimony ever could. By the same token, if no connection between the two students could be discerned, B would look better regardless of its slightly greater prodigality. If the professor was the only one who taught that class or assigned that essay topic, A would be favored much more than if the course was offered every term by different professors who assigned similar topics. I suspect that there is probably a laundry list of evidential factors that would trivialize the influence of parsimony in the final assessment.
I was recently involved in a discussion of whether the occurrence of “the worker deserves his wages” in both 1 Timothy 5:18 and Luke 10:7 makes it more likely that the former is quoting the latter than that both are drawn from some other source. I personally cannot see how parsimony gives us much more to go on than it does in the case of two students with similar essays.
To my mind, the virtue of Okkam's Razor in historiography is mostly as a guide to investigation. You should start with the simplest hypothesis not because it is the most likely to be true. Indeed, considering the complex forces that shape history, any simple explanation will likely leave much to be desired. However, it is still good to start with the simplest explanation because it will be the one that is easiest to verify or falsify as there are fewer variables for which to control. If it proves impossible to come to a conclusion about a simple explanation, the chances of having any certainty about more complex conclusions are remote.
I read a fair amount of history, but the only place I ever see Occam’s Razor invoked with any frequency is in New Testament studies. I suspect this is because the evidence is simply so sparse that there is little left to fall back on. Unfortunately, like many of the other criteria that New Testament scholars have developed, I just don’t think it can bear anywhere near the weight that they place on it.