I watched a lot of college softball this season and got to see Kelly Barnhill and Matty Moss pitch on several occasions. They are both phenomenal freshman who can throw 70 MPH, and neither one has a decent change up. When you throw 70 MPH in TB and HS you don't need a change up....but guess what, players in the SEC can hit 70 MPH pitching, and not having a change up diminished their effectiveness. As Ken mentioned, a change up does not have to be a 40 MPH pitch that paints the outside corner. It just has to throw off the batters rhythm and get into the batters head that they have to watch for it....
As I have worked with my DD to try to develop hers, the biggest challenge is to get my DD to realize that keeping her arm speed up and allowing the grip to take speed off the ball is more effective than trying to slow down her arm and guide the pitch. When she slows down her arm she loses her timing and ends up throwing the ball 38' or over the catchers head. While my DD does not throw 70 MPH, she has always relied on her full speed pitches, but thanks to [MENTION=10413]riseball[/MENTION] and others we have made developing a change up the top priority this season!
The one regret, the one mulligan I wish I had would have been to insist that she develop a lights out, buckle their knees, make them look silly change up while still in High School.