I'm speaking strictly traditionally. I'm not saying there aren't exceptions, as there are countless combinations that these boutique shoe shops have now had made up given the popularity of menswear in recent years.
However, traditionally speaking, an oil soaked sole is going to fall into a more casual category than a traditional oak tanned sole simply because of it's appearance. You are right that if the edge trim is black, then most people will never notice. However, there are still those who cross their legs and show their soles, even though it isn't "couth". It isn't a deal breaker on any shoe, and I didn't mean it that way. Rather, if I had three pairs of black captoes lined up, which were all the same, with the exception of the sole, and one had an oak tanned sole with black edge trim and black finishing on the bottom (where it doesn't touch the ground), one had traditional oak tanned soles, and one had the rich brown oil soaked soles, I would rank their formality in the order I listed them.
I was answering the OP's question on why Alden doesn't use oiled leather soles on all their shoes. It certainly wouldn't hurt to have them on most shoes, assuming you like them.
Sounds perfectly reasonable. Thanks...