Dead is dead, that that should apply to the movies as well as the comics.
The current editorial policy at Marvel comics is that no character comes back from the dead without an absolutely necessary reason.
The reason for that is that Jean has died and been resurrected so many times in X-Men comics she might as well change her name to Kenny and join the cast of South Park.
Characters dying and coming back lessens the impact of character deaths. Jean's already died once in the movies. Sure, they can do something like Phoenix wasn't really Jean, but that makes deaths in the movies even more pointless.
Technically, based on what happened in X-Men comics, Jean shouldn't have even died...
In Uncanny X-Men 208, Wolverine stabbed Rachel Summers (Phoenix II) in the lungs and heart, and she survived. (She used her TK to seel the wounds) So, what Wolvie did to Jean in Last Stand should not have killed her, if Rachel could have survived those injuries.
But clearly none of the writers of the X3 read UXM 208 or especially 210 and Ororo's lecture to Logan about what he did to Rachel...