It's anything but a "cheap gimmick" and Star Trek
TOS was virtually always "human focused" - we saw, at most, a couple of aliens serving in Starfleet during the entire run of the series, and not even that many human beings who were directly identified as having been born anywhere other than Earth.
Vulcan was talked about in TOS, but how many Vulcans did we meet? How many were important in Starfleet or the Federation?
For all the hindsight fans like to bring to this, aliens were "the other guys" in TOS, not Starfleet or the Federation.