This may have already been mentioned somewhere, but topspin and pace obviously go best together. However, it is not a bad strategy at all to primarily hit softer loopier rally shots (even at moderately high levels of play).
If your opponent can handle these shots easily something else obviously must be done, but most people will not crush these shots consistently and these can set you up for short shots that you can step into.
It is also a misconception that players with western grips hit with less pace. Western grips are great for hitting shots at 80-90% speed and power with more consistently. Players with this grip (although it is possible with all grips) can really throw their body through the ball and keep it in with more of a line drive topspin, as well as loopier topspin. It may be a bit harder to crush shots at 100% and hit outright winners, but many players would rather stay on the safer side. Not everyone (especially not rec players and young juniors) can crush shots like del potro.