Well, when you're right, you're right, and when you're not...
Had an interesting experience tonight as it relates to this thread. A few weeks ago, I noticed a dark(er) spot in my leather passenger side seat (black). I got a new brush from Amazon I wanted to try out so tonight I went out there and used the Chem. Guys cleaner on the area where the spot is and the entire seated portion of the seat. As usual I wiped it down first, then applied the cleaner, scrubbed it with a brush, and wiped it up. I just cleaned these seats the same way back in April so I didn't expect much, if any, dirt on the seat and when I wiped up the cleaner after scrubbing, the rag didn't show any. However, the spot still remained. So, I got the Meguiars Super Degreaser to let that take a crack at it. Since I thought the seat was clean, I just sprayed the section where the spot was, scrubbed it again, and wiped it up.
The cloth I used was Dirty! Not just a little, but a lot. I went ahead and did the rest of the seated portion of the seat. Here's the rag after I finished:

That's just one side of it! This was indeed a surprise. These seats get "cleaned" (or so I thought) once a quarter and have been for several years. I've worn out like 2 brushes that just get used on leather seats, though on more cars than just this one.
There's a few things I take away from this:
-- our leather seats are probably MUCH dirtier than we think they are
-- Even specific car leather CLEANER products (i.e.
not cleaner/conditioner, etc.) that get good reviews don't really clean the seats
-- retail "cleaner"/conditioner/etc. products aren't going to clean your leather seats, at least not completely
-- I will not be recommending Chem. Guys Leather cleaner anymore
What I'm calling the stain -- probably a discoloration of the coating, and
possibly brought on by another leather product (Chemical Guys Quick Detailer) -- is still there. I'll get the steamer after it in the next week or two.