Well, it only makes sense...leather comes from skin and skin is a living organ. The largest organ of the body. I've always held to the notion that if you'd feel comfortable putting it on your own skin, then it's probably OK for leather. GlenKaren is all natural if you can eat it...it can't be harmful to your skin or leather.
By contrast, I was just looking at my bottle of Venetian..."Danger: Harmful if Swallowed Contains Petroleum Distillates, Turpentine..."
Turpentine is used as a solvent for waxes, varnishes, and oils in the furniture industry. It is also a good solvent for the oils and fats that naturally reside in high quality leather. Add ed to that is the fact that it is flammable. As such it is "fugitive." It evaporates. When it evaporates it will draw conditioners to the surface and some of them will either evaporate off with the turp or be lost to dust.
In any case, the fact that it is used as a solvent for low end commercial shoe preparations suggests that its primary purpose is to evaporate and accelerate the drying of waxes and oils. So it wouldn't be too surprising if there were anecdotal evidence for drying out leather, as well.
I usually include product with any pair of shoes or boots I make--at this time it is Glenkaren and Bick4. I avoid any product that contains such warnings as those mentioned above as a matter of course. Unfortunately even if a product contains such additives, if it is not designed for human ingestion, it will more often than not have no ingredients listed or any warnings whatsoever.