JME has many geometry classes such as Vector3f, Matrix3f, Quaternion3f, etc. that are also present and similar in the Java3D vecmath package. It is certainly conceivable that another library out there has similar class names and that a developer prefers using one library or another. Does it seem like a good idea to extract interfaces from these classes so that others may implement them and use their own objects instead of what is in jME?
You could create a bridge instead ??
I think avoiding interfaces was an intentional decision in order to improve performance. Further the classes are quite specialized concerning jME's needs.