(Re: Ethan Reigns) Um, not to be Mr.Correctness or anything, but there were 110 episodes of Angel (Unless you are counting the 10 minute unaired pilot), and Buffy only had two guest spots on Angel (Again, unless you are counting her un-SMG appearance in Soul Purpose, and her stock-footage appearances in City Of, Bachelor Party, and Just Rewards).
Sorry, I'm just very literal and precise on the subject of Buffy, Angel, and all it's details.
On the actual subject in question, I think it was planned that way, because if Faith were involved in the Jasmine final arc of Season 4, she would just have been like everybody else: Under her thrall, depressed about being brought back, and probably would have left before they took over W&H. Plus, her being in the last fight on Buffy with all the activated Slayers, plus her interactions with Spike were a better place for her to be than involved with trying to stop Jasmine.