Interesting enough, suddenly the registration system over here in germany nearly does make sense when reading that. Yes, it all starts with the D, but the next thing after that actually defines aircraft class (and weight). Therefore the full callsign is helpful to give a (very) rough idea about what is in the circuit or about to join. Of course there are some exceptions, larger flight schools use company callsigns similar to airlines. At least one uses airline callsigns (lufthansa), but they only fly (small) jets in germany anyway (Cessna CJ1).