I don't believe anything should be done exclusively - I think (and this is my problem with the age group mandates in mini tennis) that as a coach you use all the tools at your disposal until the desired effect is achieved. If that requires a sponge ball and a service box for an adult then so be it!
I tweet - @ashtennis guru (no spaces)
I Shoot - www.flickr.com/photos/ashtennis guru/ (again no spaces! grrr)