Also, that old "depth is everything" advice came out in the early '70''s.
Since then, TomOkker started getting good, as did JohanKriek, and topspin started to take over. Then some BB guy came around, and while depth is great, short angles and the chance of hitting short, but pinning the opponent behind his base line came into play.
You slice, you aim for depth. You hit flat, you aim for depth. You hit with HEAVY topspin, you can hit shorter and get a way with it, and also reap the benefits of sharper angles.