The chance encounter occurred at an NAACP fund-raiser held the day after Nichols…turned in her resignation.
Wasn't she a dayplayer? Why would she hand in her resignation, she never had a contract. What's the point in resigning and ensuring they never ask you back, when she could have been unavailable for as many episodes as she liked. The producers might have decided to get rid of the character, but maybe she could have returned to the show later.
She had no obligations to Star Trek, she could try to get every job she wanted and if it had worked out she could have left the show without further notice.
I don't know and don't care if she met MLK or not, but I don't truly believe that she wanted to quit in the first place, she may have dreamed about getting storylines or finding a better job, but it makes almost no sense that she thought NOT making money while she pursued other opportunities was better than making money and having regular appearances on a network show on her resume while she's doing it.
In fact, all of "the other four" got gigs on other shows while Star Trek was still on the air.
I suspect Nichelle was up for the part of Peggy, Mannix's secretary. That definitely would've precluded any more appearances as Uhura, or at least a substantial number of them.
Who knows, we have wound up praising Elizabeth Rogers for those hundred or so appearances as Lt. Palmer, instead of just the two.