For me, this design answers an old question about the nature of the Aurora: was she a hypermodern and luxurious yacht arrogantly stolen from top Catullan bigwigs, or a rusty old bucket commandeered from some disreputable corner of Catulla's dirtiest starport? The VW Kleinbus appearance speaks for the latter...
This could easily also be what the Denevan ship was like up close. It doesn't make for a very good vehicle for Harry Mudd, though, as "freighter" would imply something a bit more substantial. Oh, well, I can live with that. And it does make more sense that Mudd would appropriate a Kleinbus than that he would come to possession of a locomotive pulling a freight train.
And if any vehicle in TOS can be expected to make multiple appearances in various civilian hands, a low-cost personnel mover like this is the best candidate.
Timo Saloniemi