Whenever I'm at my favorite Kansas City gym, I still see lots of members sitting and standing around waiting for the next set. Either they are training for a body building competition or they don't know that resting between sets is out of style!
In a time-crunched world, who has time for rest during workouts? The old-fashioned "do a set, rest, do a set, rest", and so on is effective, but it isn't efficient when it comes to overall fitness. If you're looking to build muscle by doing low reps, high intensity sets, keep doing so. To maximize your time and continue to improve other aspects of your fitness, such as core movements and flexibility, choose a low intensity exercise to do in that 45-90 second rest period that doesn't elevate your heart rate. This might include a plank or a dynamic stretch.
Whatever you do, keep moving!