I think this is a good rule, especially for girls basketball. Hopefully coaches will use is properly and not abuse it too much. The issue is many JV games are going to 2 quarters or cancelled altogether because of lack on numbers. With the additional quarter, you could play a full varsity and JV games with as few as 8 players. Now that takes into account everyone plays full quarters with no breaks and no one fouls out or gets hurt. Varsity players would have to play 1-2 quarters of the jv games. My guess is this probably still wouldn't happen, but it at least opens up the possibility.
The issue has become that some of these programs that struggle to get more than 10-12 out for basketball has limited or no JV games, and that's a program killer. Those freshmen/ sophomores who aren't varsity level players never play and subsequently stop coming out. You need some kind of JV or reserve program to get kids playing time.