I was watching the episode from Season 3, "Dark Sky/Diaspro's Deception", and V/Baltor is impressed by Diaspro's success on spelling Sky. He even knows that she could make an excellent edition to the witches' team. The Trix were uneasy about this, since they probably didn't really know her very well (or even liked the thought of adding people to their trio). V/Baltor wasn't too serious about that thought, though. Since Diaspro wasn't really shown much in collaboration with V/Baltor after episode 9.
But now, in Season 6, we see that Diaspro and Trix join forces to take down Bloom. Although we have no idea what happened to her after blocking an attack from Daphne. Nor does she appear with the Trix or Selina after that. But I digress.
Perhaps it was in the writers' minds to have this devious princess join the Trix. Ever since season 3 at least. Or dare I say Season 1, when Bloom though Diaspro was Icy (or the other witches) in disguise.
It's very interesting to see how everything played out in the end. Part of me is a fan of bringing together the bad girls of Winx Club, and to team up together. Well, we do have four female baddies that are overused a lot. Doesn't surprise me that Rainbow had the Trix and Diaspro come together eventually.