The fact that they appeared is secondary to how they were treated. Kang, Kor, Koloth, and Arne Darvin were all on DS9 after Berman had turned over most of his control to Piller and eventually Behr.
Let's break the others down:
Chekov: was only featured because Koenig agreed to take the part that was written for Kelley. His lines and function in GEN were unchanged from what was written for McCoy. I'd say that's poor treatment at best and insulting at worst.
Scotty: same for Doohan, who took the part written for Nimoy. Some of his lines and his function in the story were unchanged from what was written for Nimoy. This is also insulting.
Meanwhile, Scotty was treated horribly in "Relics." It's an insult to his character and makes the TNG crew look awful. Berman had a big part in that.
Spock: Spock, perhaps the greatest Trek character after Kirk, was criminally underused. The progress he had made with Sarek in TOS and the TOS films was tarnished. This is more poor treatment.
Sulu and Rand: they were treated better than the other TOS characters because it was an anniversary episode and because the episode had to maintain continuity with Star Trek VI.
James T. Kirk: the heart of Star Trek was treated like a plot device. He was sidelined after his first unnecessary "death" until he was unceremoniously killed again.
McCoy: he was treated best of all because Roddenberry was involved with and co-wrote "Encounter At Farpoint." He was never seen or referred to again (even though it would have made sense to do so when Spock and Scotty appeared).
Berman hated TOS. If you need further proof, spend some time reading background information about TNG, Berman, Braga, and Ron Moore on Memory Alpha.