Yes you are right.
There are actually 2 types of repositories in an SDE Instance.
The SDE Repository (Tables with SDE_ Prefix) store which tables and views hold spatial data and some basic infos about them (Metadata, Indexes, Schema, Storage type). you can register simple (OGC SFS) vector functionclasses or rasters (raster dataset). This exists since the very earliest versions of ArcSDE 3.0.
The Geodatabase (Tables with GDB_ Prefix) where introduced with ArcSDE 8.x (major update in 10.x) and live on top of SDE. So registering with Geodatabase, alos registers with SDE. Geodatabase brings in some additional advanced features like
- storing splines, cad data, annotations and so on
- cartographic representations
- mosaic dataset
- and much more
If you don't need any of these functions you also do not need Geodatabase. In most cases its enough to just register with SDE so you can view (and also edit) your feature classes in ArcGIS