Harvey needs Donna, and he needs Donna to be cheekily sitting at his desk when he walks in.
There have been romantic—or romantic-seeming–moments between the two characters, most notably when Harvey said, “You know I love you, Donna,” after he got her court case thrown out.
Harvey’s always been fond of big and grand gestures in clutch moments, but Donna is important to Harvey on a different level.
I know they had that one time, and I know if they did get back together, she wouldn’t work for him.