Ahh. So the feature is compiled in. As such digikam expects marble to be in place. Then Patrick I would have to agree that it be included. With all due respect; for those that would be relying on the feature it would be sorely missed if it wasn't there. And then we would be complaining that the feature wasn't included.
Maybe you should try to communicate with the developers and ask them if there is any way to get digikam to compile with support built, check if the package is installed and run even if the package is not installed. That way the dependency can be removed. Until such time Fedora has no real option but to include it.