I'm looking for balance, here. Let me provide an example:
I have a "left-swipe-to-delete-from-list." That's the standard way that Apple has decreed that items should be deleted. They provide no affordance.
You can add an "Edit" button to the navbar, but that means you don't have room for other, more important (and frequently-used) items. In my case, this is a non-starter. I need the room for more important stuff.
So far, I have added the left-swipe, but no affordance. It relies on the fact that the platform standard is left-swipe to delete. I need to make sure that the screen shows a fairly standard list (not getting too fancy), so users that are trained on platform standard will know that they can left-swipe.
In some cases, affordances can actually interfere with usability.
I can develop my own affordance, but I am not sure what a suitable one would be.
Implied training is also a big part of usability. I think it was Tufte that talked about that. He has some really strange UIs that don't make sense, until you learn them, and then, you don't ever want to go back.
In my experience, train maps in Tokyo are like this. They are a fearsome mess, when you first look at them, but, once you understand them, they are marvelous.