Primmin wasn't meant to be a permanent character, I think. He was written in to fill the void when Colm Meaney had to be away from the show for a couple of weeks.
There was a lot of good stuff in the first season of Men in Black: The Series (the animated sequel to the movie) that was abandoned afterward. In the first season, it was a very smart, character-driven show, and the agents had engaging personalities and interactions. But after the first season, most of the writing staff moved to Godzilla: The Series and got replaced, and the sophistication of the show took a nosedive. The second season had some decent plots, but they were driven by action and gimmicks, and the intriguing characterizations were lost. And as the show went on, it came to be dominated more and more by silly gimmicks and annoying comic-relief aliens.
Also, in the first season, Agent K (Tommy Lee Jones's character in the film) was engagingly played by Ed O'Ross, who sounded uncannily like Joe Friday but with elements of Jones's performance in his characterization. In the second season, he was replaced by Gregg Berger, who did a much, much blander, more robotic, and rather irritating voice. The character designs were also changed to be simpler and cleaner, but K's design was changed so much that, along with the voice and personality changes, he didn't even seem to be the same character.
Much the same happened with Agent L between the third and final seasons: Jennifer Lien was replaced by a different actress with a less appealing voice, and L's character was radically altered, going from the ultracool, sultry, and savvy agent she'd been to a perpetually angry and flustered second banana to a newly introduced alien MiB agent.
An even more drastic example is War of the Worlds: The Series. After the first season, the show changed producers and underwent a drastic, wholesale retool that destroyed essentially everything worthwhile in the show. The first-season cast was fantastic, the best thing about the show, with a great rapport and chemistry among the four leads. The new showrunner killed off half the cast, including the most popular cast member. (Why? Well, it's worth noting that the characters he killed off were a Native American and a black paraplegic, while the two white regulars survived and a new white guy was added to fill out the team.) The surviving leading man, who had been charmingly eccentric in the first season, lost everything that made him distinctive and became entirely bland and generic. Virtually everything about the aliens and the storyline that was based on the 1953 WotW movie was dropped in favor of a new bunch of humanoid-appearing, religious-fanatic aliens. The world of the show was inexplicably changed from a fairly normal present-day setting to a dismal, depressing post-apocalyptic environment. And while the writing on the first season had been uneven, the second season's writing was mostly awful, at least until about halfway through, when it started to rally a bit -- though it plummeted again in the series finale, which gratuitously retconned everything we'd been told about the aliens since the original movie in order to tack on a happy ending out of nowhere.
