If FC had been a Star Trek mini-series or something spanning multiple episodes, I could see bringing in characters like Sisko to flesh out the story. But for a single film, adding Sisko (a character that many TNG fans might not even know about) would have just overcomplicated things.
That didn't stop the Admiral Janeway cameo in Nemesis. That could have been any admiral. I wasn't talking about making Sisko a major part of the story, but wild horses wouldn't keep Sisko away from a battle with the Borg. He not only has issues with them but he also helped design and build Defiant. He could have stayed with the ship while Worf went off to help out on Enterprise.