It was acceptable to bring Spike back for the movie, which took place before the end, but sometimes things end so well that you really don't need any more. The show explored all the characters, so to make more BeBop is to basically keep putting them in superfluous adventures.
As for "It's all just a dream..." Well, there's the issue with Spike's eyes, but it goes deeper than that. I interpret the "dream" as being the time Spike spent aboard the Bebop. His encounters with Vicious and love for Julia, however, compel him to confront his past and to take on the syndicate at the end.
Remeraber, Spike's plan was for him and Julia to escape the syndicate together, but that went wrong. Ironically, he ended up living his independent dream but without the one he wanted to share it with. So his comment to Faye that one eye sees the past whereas the other sees the present could be seen as a direct metaphor: his "dream" is the conflict between the free life he's living in the present and the past he couldn't let go of, which eventually caught up with him. Julia had the same issue. She was given a choice: kill Spike and live in the present, or love him and erabrace the past that they had.
The past and the present couldn't be in conflict forever. The end of the dream was the mutual decision of Spike and Julia to confront the past instead of trying to avoid it. Then Julia dies, which makes his choice absolute. He goes back to Bebop to basically say his final goodbyes before he settles things for good. He felt that he couldn't live free without killing Vicious and cutting his ties with the syndicate forever. Hence this final line to Faye: "I'm not going there to die. I'm going to find out if I'm really alive."
Not that the message neeRAB to be seen as absolute. Was this really Spike's inevitable tragic fate, or could he have chosen to forget the past? Should he have? Was Faye right in the end?
Aaah, yeah. There's a good reason people still love Bebop.