On the rarity of human rebirth....
When you consider all the possible beings in all the different realms of existence, [some very large number] and you consider the possibility of them being born into one particular realm, like the human realm [some significantly smaller number of beings] then indeed, rebirth from one of those realms to the human realm would be extremely rare.
Though I do think there is evidence (Canonical I mean, not anecdotal) that a human has a pretty good shot at being reborn in that same realm. Of course we know this is the fact in the case of anybody with any sort of attainment (stream entry or above). There are cases in the Canon of the Buddha himself recalling past human lives. The Buddha talks of the causes of people being born into fortunate or unfortunate (human) circumstances as a result of fortunate or unfortunate (human) past actions.
My reading of suttas such as the Kukkuravatika and the Sankhārupapatti Sutta
tell me that we can directly influence our rebirth by our actions in this life, and that a lot of it has to do with ones state of mind - dog state of mind, ox state of mind, human state of mind.
So, rare in general, but perhaps not so rare for us who area already in the "human club." That's just my opinion based on my interpretation of the above.
Any "increase" we see here is a result of a combination of factors, of which kamma is only one.
As far as evidence collected from people recollecting this or that, I find it intriguing, but it doesn't do much to support rebirth. I could come up with a number of equally wildly speculative explanations about why a person would remember such a thing. I could also probably come up with some rather mundane theories.
As for the usefulness of pondering such a thing?
It can be a bit of a diversion. But it may help us understand the law of kamma a little better.