I think some people are just story purists. I am kind of that way about the Lord of the Rings. I try to like Two Towers and Return of the King movies but a part of me can't help but hate them a little for diverging from the books so much. I can't think of a good reason for those divergences other than the arrogance of the writers and directors thinking that they understand the character motives and feelings better than Tolkien did.
For me Comics have always had characters coming back from the dead. Sure a few stay dead, but the majority come back at some point or other. Sure the original Gwen Stacy is dead, but now we have the popular Ghost Spider version of Gwen from an alternative universe that got super popular during the first SpiderGeddon crossover.
I like Jean Grey, I like the dynamic between Jean, Rachel, and Cable when it is properly written; and I would love to read a comic that explores that. I would love to read a comic where Jean and Emma are forced to work together on a mission because I feel like the dynamic between them is fun to read and I would love to read more of it. Heck I could read a comic where Jean and Emma co-headmistress a school just for the fun of having them argue on a regular basis.
I like Jean for herself too, I thought X-Men Red really captured her as a character. There was a lot of filler in the X-Men Red story, the Cassandra Nova thing dragged on and on and I wish they had broken the series up into some story arcs instead of having one long arc revolving around Cassandra Nova. But I loved Jean in that series.
I liked the Jean Grey series, particular the last few issues where Emma and Ghost Jean are working together. I found the Return of Jean Grey to be a bit meh, but I guess I felt like they are trying too hard to keep Phoenix as some malicious entity when I think Phoenix just feels things very intensely. Once again that wasn't Jean's fault, she was fine in that series too.
So I can understand why some of the "purists" don't want Jean back, but I prefer having her in the X-Men comics because I think she represents Xavier's dream better than Xavier himself does. I actually couldn't care less if Xavier ever comes back permanently, I would rather have Jean back as one of the core leads of the X-Men.