But that doesn't hold true for other fandoms, like due South or Starsky and Hutch or The Sentinel (trying to reference once-large but still active fandoms of the same vintage). They all had small if not smaller core groups of characters, and (to my taste) far less interesting minor characters with recurring roles (Fitz!). With Highlander, a single appearance was enough to establish some characters as fic favorites -- look at Matthew McCormick, Corey Raines, and Byron; and look at what three shows and a flashback did for Kronos, to say nothing of the Horsemen in general. Duncan's large extended family gives us a great deal to work with, even when characters have a small part (Darius; Grace; Father Paul; Kenny; Ceirdwyn; heh, Bennie!!) as do the associates and the backgrounds of the Immies who wander in and out. And the Watchers have their own history to be explored and their own social life and the complete backstory for Adam Pierson and his friends and his bosses -- in two universes, yet. We have tons more material here than some of the other shows that aren't in the middle of a crossover crisis.
OCs are plentiful in HL stories; they're unavoidable for historicals or for stories set in the future or stories that deal with Joe's past or the Watchers or for anyone's backstory. I don't think there's a real resistance in the fandom to OCs in general if they fit into the story -- obviously, since some of them are quite popular on their own and writers are praised for creating successful ones. What readers do tend to resist are OCs created only as a mate for one of the top men (the whole reason behind Poor Roger was to get readers to cut the poor guy a little slack). It's not impossible to win them over -- there was a series of stories about a character in a BDSM relationship with Methos that was popular, even with Duncan being marginalized -- but it's much harder to pull off.