Since looking at this thread, I began daydreaming what it would have looked like, if, instead of Spock going through the orifice alone as in TMP, what if Kirk and Spock and gone through together?
I'd say it would have worked much better. The problem is that when Douglas Trumbull took over the special effects he really didn't have much time left. So he had to find a way to simplify things, without diminishing the movie's visual impact, and having Spock going alone was certainly simpler.
This is something I've always found funny. They had 10 years to make a movie and still they end up doing it in a rush. I remembered when the Star Wars DVDs were released, one of the people in charge said that their main enemy has been time. So the movies were released at least 20 years before, Star Wars was absolutely the last franchise to be released on DVD and still they did it rushing. It must be part of the nature of man: always allocate less time than necessary to do something.