I don't get the "either/or" mentality. It's perfectly normal to film two things at once and enjoy them both. Easy for Takei to support TOS even while on other projects. One experience doesn't detract from the other.
Exactly. Unless you're a series regular (which, remember, Takei wasn't), you can't make a living as an actor without taking multiple jobs. It's common for recurring cast members on one series to take gigs on other series during its run.
In fact, let me research that -- other acting jobs by TOS main cast during TOS's run.
episode "Quaker Girl"; movie White Comanche
; CBS Playhouse
episode "Shadow Game"; TV movie The Skirts of Happy Chance
Nimoy, Kelley: None I can find.
Doohan: Iron Horse
premiere episode "Rail Runs West"; Jericho
(1966) episode "Eric the Redhead"; movie Jigsaw
. (IMDb also credits him playing a recurring chauffeur on Peyton Place
at the time, but Wikipedia seems to say he only played the role before TOS and the character was then recast.)
2-parter "The Deadly Silence"; movie Doctor, You've Got to Be Kidding!
Takei: Mission: Impossible
episode "The Carriers"; Felony Squad
episode "A Blueprint for Dying"; movies The Big Mouth
and The Green Berets
; It Takes a Thief
episode "To Catch a Roaring Lion."
episode "Delayed Action."
Barrett: Please Don't Eat the Daisies
episode "Just While You're Resting"; movies A Guide for the Married Man
and Track of Thunder
; The Second Hundred Years
episode "Remember the Maine"; Here Come the Brides
episode "Lovers and Wanderers."
Okay, so Takei did more guest appearances elsewhere than most. But considering how few Asian-American actors there were in the business at the time, it stands to reason that each of them would've been in fairly heavy demand.