The established basketball schools can do it however they want because they got guys sitting on the bench who are good and unlikely to leave in-season and if they transfer...no worries they are easily replaced.
But the lesser schools, the places where programs are not established....the places that attract lesser talent: you play 9 guys and this allows you to develop people throughout the year, it minimizes the importance of injuries because you already have a 9 man rotation....you absorb the loss. When you play 6 or 7, an injury kills you...two injuries...shut it down. Your replacements haven't played...they frustrate the guys who have been playing because the fill-ins don't know how it all works....they have no confidence, they are nervous and uncomfortable.
Play nine and sell the team thing, SELL you are playing harder than the other team because we play 9 and so your defensive intensity is greater than the other team. Don't coast, don't rest on the court and become known by everybody for playing harder fthan the other team...it becomes something you can hang your hat on. People respect you. The sum is definitely greater than the parts and people feel good about it.