Rebuttal: That's not necessarily true. Several things mitigate against it being true.
First, in almost all situations, shoes crack at the ball of the foot first--where the leather is flexing the most. Why is that, do you suppose?
The reason is that oils (in conditioners) and waxes tend to collect micro fine particles of grit and hold them in place. In the creases of a shoe, these particles rub against each other and the leather itself. The grit is loosely bonded to the leather forming a temporary sandpaper, in effect. Over a period of time...esp. with the addition of more oils and waxes...this drives the particles deeper into the pores and the grain surface is cut. Each tiny defect in the grain, caused by the action of the grit, becomes a recess in which more particles can build up and work their way deeper. As well as water (which can change the ph of the leather) and other deleterious substances.
We tend to discount these events because they take place on such a small scale and over such a relatively long period of time...slowly, slowly. We don't notice until it is too late.
Second, the finish on the leather, as thin as it may be, is still an extra layer that is fundamentally more fragile than the grain itself, in most cases. And in most cases the grain has been "snuffed"...if only slightly...to allow the finish to adhere. Finished leathers tend, IMO, to crack more quickly than crust leathers although the difference may be unnoticeable in the normal course of events.
None of this has anything to do with the moisture in the leather per se except that dry leathers will, of course, absorb conditioners faster, and carry grit deeper into the leather. Added to that is what you alluded to--some leathers, dried out leathers in particular, are comprised of fibers that are more brittle than other leathers. But it is worth remembering that some leathers are drier than others right out of the tannery. And those in cases where the leather has "died" or been affected by red rot, no amount of moisturizing can bring it back or prevent cracking--often catastrophic cracking.
This question was first posed and answered in our Official Shoe Care Thread.
Have a question and can't find an answer? Try here: http://www.styleforum.net/t/228153/the-official-shoe-care-thread-tutorials-photos-etc