It's a bit like TMP is trying to appeal to the fanbase, maybe a bit like those television reunion show specials. You know the sort of thing: "Hey look folks, the whole gang's back together again! Even Janice Rand is here!"
Which is actually one of the reasons why the film is criticized. As much as fans liked seeing the crew reunited, a lot of people have since observed that TMP suffers from trying to make it appear as though only a short amount of time has passed since the end of the five-year mission. The problem is that over ten years have passed for the actors. The cast was clearly aging. Shatner had a toupee.
Harve Bennett and Nick Meyer decided that aging was one of the themes they would embrace for the second film because it fit more with where the actors themselves were in their lives and was a much easier sell for the audience.