Well ducal capitals have a number of problems, namely that there isn't, currently, any such thing. You need a city or stronghold to create a duchy, but you do not need to have either of these for a duchy to continue existing. These region types are currently the only ones that can be capitals, and thus allow recruiting (and banking), making them extra valuable. And even when a duchy has at least one, it can have more, and it doesn't mean the duke has any such regions, if any region at all.
I don't think those issues have ever been properly addressed in any feature request. If someone can flesh out a system that would make ducal recruitment sensible, I'm sure it would warrant at least a discussion.
