The D-League has 18 teams, and 17 of them have single-affiliation relationships with NBA franchises. The lone completely independent organization in the D-League is the Fort Wayne Mad Ants. If the goal is for every team to have an affiliate, there will be more expansion in the coming years.
It would obviously benefit the Raptors to have a team of their own in the D-League, whether it is owned and operated by Maple Leaf Sports and Entertainment or a hybrid model where Toronto would control basketball operations but not own the team. By now we've seen example after example of NBA teams using the minor league to groom players, coaches and staff.
Outside of a team located in Ontario, one in nearby Rochester is about as convenient a situation as the Raptors could possibly hope for.