I just find it interesting because as a kid growing up in the late 70s and early 80s I watched a lot of 60s re-runs and I've never seen so many TOS guests. In Gilligan's Island for example, I can only think of one, which is Stanley Adams (Cyrano Jones).
Mickey Morton (Kloog) was in the same episode where Adams appeared. Also, Michael Forest (Apollo) played an island native in the Gilligan's Island
episode "Slave Girl."
Some more obscure connections: Billy Curtis, who was one of the fez-wearing gold-skinned ambassadors in "Journey to Babel," appeared in a dream sequence in "Lovey's Secret Admirer." Janos Prohaska played a gorilla in three Gilligan
episodes (and probably any other show that featured a gorilla in the '60s). And Charles Maxwell, who played Virgil Earp in "Spectre of the Gun," was in more Gilligan
episodes than anyone but the seven leads, though he was never credited; he was the voice on the radio.
And he's not a TOS guest, but Frank Corsentino, who played three different Ferengi in TNG and VGR, made his screen debut as a "Native" in a GI episode. Also, the first Gilligan
reunion movie had Vincent Schiavelli (TNG: "The Arsenal of Freedom") and the third had Rosalind Chao.
Not to mention that GI's music for most of the series was composed by Gerald Fried, best known for scoring "Amok Time." (Before that it was by Johnny Williams, better known today as John Williams.)
Maybe what's surprising is that GI guests like Hans Conreid, Vito Scotti, Nehemiah Persoff, and the teenaged Kurt Russell never did Star Trek
when they were so ubiquitous on '60s TV.