Occasionally, yes. But it's far, far more wrong to refer to Takei, Nichols, and Koenig as extras.
Well, they're extras the vast majority of the time. Occasionally they'll get some mild focus in ship-centric episodes like The Naked Time or Mirror, Mirror but that's because there's no significant guest stars around. If there are, they're given nothing to do really.
Being considered extras is only wrong if people look down on extras. It's a perfectly valid job role that TV and film would be much worse off without. Not everybody has to be, or has the talent to be, a star. As George Takei's performance in his Voyager appearance proved.
Yet these people feel entitled to fame, think they deserve to be in the public eye and demand the attention that they feel is being robbed of them in favour of Shatner. That's just... wrong. It'd be like the guy who played Uncle Owen in Star Wars complaining that Mark Hamill hogged the lime-light. No, he's just the bloody main character. That's how fiction works.