If the resource dictionary implementation works for you, I would advise that you continue to use it. The only drawback it has is that it does not support Blend style/template editing which is crucial for our controls.
That is why we have implemented a XAML preprocessor that is able to merge files at compile time and produce a single XAML resource that Blend can "see". The preprocessor does not come "for free" -- to use it, you would have to modify your projects' MSBuild code to include the custom tasks and targets, which is not for the faint of heart. If you really want to see how we are doing it, open a .csproj file in the source distro and search for the PreprocessXaml
the Telerik team