But was there also something of a deliberate decision in Trek not to go for big-name guest stars, the way that, for example, Batman did, so as not to have the stories overshadowed?
I've never heard anything to that effect. And they definitely weren't above the occasional celebrity stunt casting (Melvin Belli certainly wasn't cast for his acting talent). Not to mention that some of Batman
's celebrity villains did in fact appear in Star Trek
, including Frank Gorshin, Julie Newmar, Lee Meriwether, and Joan Collins (as well as lower-tier Bat-villains Malachi Throne and Roger C. Carmel).
Gorshin did occur to me (I'd forgotten about the other Bat-villain appearances). I don't know if Collins was a stunt-cast - was she as well known in the 1960s as she later was?
But I'd say you're right, it wasn't a deliberate policy. It's probably just a happy accident that, all those years later, we're not distracted by big-name stars, simply because the show couldn't afford them.