Apparently - around the time of Tribbles, he had been in a series of beer commercials as - yes - a bartender. but his IMDB listing long and impressive
So many character actors don't get enough respect. Many of them are very, very good!
The thing is, most of these people played basically the same types of roles - and they did them very well. Look at Trek:
Roger C Carmel was really the only choice for Harry Mudd - Kandel never said who he had in mind when he wrote it, but admitted that he really WAS Mudd.
William Windom made a career out of playing normal guys who go very close or over the edge.
Robert Walker Jr was the epitome of adolescent/young adult angst in 60s TV.
and so on...
Most casting directors for TV in the 60s/70s were casting multiple shows, so yes, you would tend to use familiar faces you know can do the job.