There is a logic at work here: the logic of so-called logical relevance, which treats evidence as relevant if it has the slightest amount of probative value, if it has any tendency to increase or decrease the probability of a legally-material fact. See Federal Rule of Evidence 401. The same logic is corroding the doctrine of chances and the habit rule. In the case of the doctrine of chances, it is not apparent why more than just other other prior "accidental" bathtub drowning or more just one other supposed SIDS death is necessary to make the other drowning or the other infant death admissible in, respectively, a prosecution of accused for deliberately drowning his or her spouse or a father or mother for deliberately smothering an infant. And in the case of habit, it is hard to explain, under this logic (the logic of "logical relevance"), why a pattern of behavior must be invariable or almost invariable if the that pattern of behavior is to constitute habit legally speaking and thus avoid the prohibition against the use of character to show conduct.
It is this same sort of logic, I think, that drove the Supreme Court of the United States to hold that "clear and convincing" evidence of the commission of another crime is not necessary if another crime is to be used for a "nonpropensity purpose" such as "knowledge" or "absence of mistake" and that evidence "sufficient to support a finding" of the commission of the other crime is sufficient for admissibility of other crime evidence in federal trials. See Huddleston v. United States, 485 U.S. 681 (1988). The problem with this tendency (if you think it is a problem) is that if continued, it leads to the annihilation of all rules of evidence whose reason for being rests on their capacity to increase the accuracy of inference because of the accuracy of the generalizations those rules harbor about nature and humanity.
When I think about such questions, I tend to think that the fundamental questions about the reason for the existence of many rules of evidence have barely been posed, let alone answered.
coming soon: the law of evidence on Spindle Law