Would Nightwing on a Superman book be a boost to Dick or Clark? Being a under the Batfam, Dick has carried solos, team books and the main Batman book. Supes however, goes from one arc to the next, from writer to writer with loose continuity between arcs and reset buttons to give new writers a clean slate. It just seems like another instance of the Bat-Brand is taking over the DCU.
Damian was put on the TT (with characters from Dick and Tim's generation) because the book was underwater but Damian is popular. The new JL Dark DTV movie will have Batman as a member, even though the comic never featured Batman on the team and Bruce firmly doesn't believe in magic. Also the sheer number of Batman books printed at one single time.