No one denounces their contribution to the show.
But at that time why should Shatner have cared about a group of day players that could have been on another show filmed for another studio the next day?
When you're a black woman in the 1960s, what work
was there to take? There wasn't really that many shows in the 60s that featured black actors or had opportunities comparative to a white actor.
Yeah, maybe in the case of Doohan and Koeing there was work. They were white actors and could have easily got work on a Western or something on the side of Star Trek. But what about Nichelle? Takei had an advantage, there was still a lot of films in Hollywood about the Pacific Stage - there was
some work out there for an Asian male. Nichelle had
nothing (and still was willing to give it up due to her treatment on the set).
You gotta look at this from all angles. It's not a black and white "this is what they say on the casting sheet/payroll" - there are other factors that play into this and to not take those factors into consideration is a dick move. Nothing here can justify Shatner's actions towards his co-stars, even if he did not mean it maliciously. His lack of concern in return hurt his costars.
And in the same light, nothing can condone the over zealous bashing on part of their end. But there is a cause and effect to everything and it must be acknowledge. Shatner isn't simply the victim here.