Gotham Central wrote:
1) Why weren't more of the aliens depicted in the RecDeck scene used in other places in the film? The Rec Deck scene gives the impression of a fairly diverse crew for the Enterprise, yet once she is underway, just about all of the aliens disappear
In addition to the other explanations offered, it was stated that many of the people wearing those costumes were not really actors, but well-known fans (ie. Bjo Trimble) or authors such as David Gerrold. These people were given a small part in the movie as a "thank you" for their years of support.
As to why nobody else ever wore those costumes... maybe they were constructed to fit a particular person?
None of those people played aliens, so it doesn't really answer the question.
Many of the costumes used in TMP were reused as the Starfleet jumpsuits and Regula One costumes seen in TWOK.