Part of the whole fun and experience and cemented-ness of Batman is the Batman Family. It would feel like serious deprivation to allegedly let Jace be his own Batman in his own city but not let him have/develop his own Batman Family with their own dynamic between themselves and Jace. Jace being different then Bruce means his Bat-Family would be different, and there is just so much potential and strength that can be mined from that alone.