Yeah, that was a bit unclear. I meant you don't prepare them as part of your normal list. You only have to make the call between them and one other option at every level, and since most domains have at least a few levels where the spell selection isn't great, you usually end up with Luck.
(For instance, one cleric I'm playing has Luck and Healing. Obviously, there's little to no reason to take many of the Healing domain spells, since I can swap out for those...)