Why did they even bother bringing him back?
Because Sisko as a character would want to be there as a father to his daughter, perhaps?
Honestly, what you see as a 'waste' of a character, I see as the natural evolution - the man was planning on building a house on Bajor that Kassidy went and built while he was gone. He is now 'of Bajor,' much like the Prophets which he spent time with.
Besides, he's only been back for... I think six months or so, in-universe, post-finale DS9 novels suffering unavoidable delays that stalled the line - there are only six books in the line that follow on from Unity, and last three of them, Warpath, Fearful Symmetry, and The Soul Key all take place close together, timeline-wise, but had significant real world time pass between the first two. Without that publishing stall, the storyline would probably be further on and Sisko's role would have had an expansion.
Sisko was offered a promotion to Admiral, one that, according to Unity, I believe, was going to be offered to him at the end of the war had he not disappeared, but turned it down to stay on Bajor with his family. I believe his official status is 'personal leave.' At this point, given that Sisko spent time with the Prophets, who do not exist in linear time, it makes sense that he would be somewhat 'outside' of events - he's been outside of time, and needs to readjust to living in the mortal realm.
The series and characters moved on in his absence - returning Sisko to the center chair just because he returned would NOT be natural, because the rest of the world has moved on.