Christian Slater's cameo has the effect of taking you out of the movie. First you hear the voice and you know it's familiar. So you're curious who it is. Then you see him and you realize "Oh wow, it's Christian Slater, the Jack Nicholson wannabe!" I loved seeing Sulu put him in his place though.
As for Kim Catrall, I read that she was Meyer's first choice, but passed on it. Or maybe the studio vetoed it. Not sure. I'm glad Kirstie Alley asked for too much money. At that point in her career, she was too famous and would've been a distraction. I also worry that her being in the movie would've forced the writers to give her more screentime. But it would've been quite a turn of events to have Saavik be the traitor. I personally have never cared for Catrall. People know her now from Sex & the City, but it seems like she's always had sex on the brain. Supposedly, she did a bunch of risque photos of herself while on the Enterprise bridge, something that incensed Nimoy. Not sure how true that part is. But I did see her doing an interview on Headline News to promote the movie where she talks about her mind meld with Spock as if it's orgasmic. Really? Can't you talk about something other than sex?