I don't feel the Cassie scenes are unnecessary at all.
I think Ennis responded to her advances because of his growing paranoia. In the scene before we are introduced to Cassie, Ennis asks Jack if he thinks people know and how he feels watched. I think when Cassie approaches Ennis, he sees this as an oppotunity to quench the rumors he thinks are making the rounds. However, he later dumps her because he either is too numb after the last scene with Jack at the lake or because he doesn't want to use her like that and end up hurting her like Alma ("I propably wasn't much fun anyways").
I agree the Cassie scenes are crutial. However, I have a slightly differnt take. By the by, I think the "dumps her" scene is
before the last scene at the lake which means that he was already moving closer and closer to a commited gay relationship. in fact his own family and Alma WANTED him to get married again, so he had every opportunity to play it "straight" if he was so inclined.
Back to Cassie scenes in general, I think her character shows how vulnerable a gay man can be when a woman makes approaches to him. I do not think Ennis saw her as a opportunity to squelch fabricated rumors. Rather, he seems "forced" into accepting her advances because he probably fears that his turning away would get the response "what's wrong with you?". Nearly every gay man I have ever known has gone through the fix ups, dating/mating game, office party hook ups with women etc etc and they went along for a while so that they were not wondered about. Close to the same point you made, 3of19, but Ennis' actions were responsive rather than proactive.
then, at some point, he simply stopped because, I am sure, the sex and commitments got too involved, and regardlless of what pop culture says about bisexuality and multiple relationships, they are
tough to handle!

Further, I think Cassie represented the "final chance/opportunity" for Ennis to break away from Jack. Ennis chose Jack. This is in spite of Ennis' speech in their last scene in which he bemoans his life. The opposite behaviors, the contractions, the inner debate, and his difficulty with the struggle for his one true direction all endorse and support the wonderful, horrible life of Ennis Delmar.