Uhura, Chapel, and M’Benga were supporting characters in TOS at best, and were horribly underutilized the first time around. Spock was a main, but one main out of three doesn’t feel like much. Kirk makes it two of three mains.
I think the main problem with Kirk, though, is that he kind of distracts from the regulars. Making him a once-a-season character, like Q in TNG, would probably work better than his three appearances during this one season.