Fan-wankery-nothing more. As somebody else pointed out, Sisko's inclusion would have been a story dead-end. And it would've damaged the characterisation of others (Worf, Lily) who the film did a good service to.
I am relieved this thread didn't start with "My friend and I remember watching First Contact in the cinema and Cpt. Sisko was in a few scenes, does anyone remember...?"
Just yanking your chain, los2188 :-)