It is very beneficial to perform many repetitions of swim drills that reinforce effective technique. (Reminders before each rep and feedback after each rep make drills even more productive.) However, there are many drills that are counterproductive and reinforce ineffective (and sometimes injury producing) technique elements.
There are numerous popular drills for freestyle. One example is “catch-up” drill (where one arm is held stationary in front of the body at the completion of the entry while the other arm recovers). This drill orients the arm in a weak and awkward position to begin the pull. In addition to the fact that catch-up stroke is counterproductive from a mechanical, physiological, and anatomical basis, extensive research by a group of scientists at the University of Rouen in France has shown that fast swimmers (or swimmers trying to go fast) do not use “catch-up” coordination. So practicing “catch-up” also has negative implications from a skill-learning perspective. (“Kicking on the side” and “one arm freestyle” are similarly counterproductive.)
Some swim drills are not counterproductive, just not very productive unless a swimmer focuses on specific cues. For example, the breaststroke “three second glide” drill presumes that swimmers will focus on their streamline (for three seconds). However, visual cues (such as looking at the bottom of the pool directly beneath the head) and kinesthetic cues (such as feeling the upper arms squeeze the ears) improve a swimmer’s focus and make the drill more productive. (There are at least seven more cues to optimize the streamline.)
Swimming drills are most productive when they isolate specific technique elements. Unfortunately, these same drills often have a logistical trade-off in that they consume more workout time. For example, breaststroke and butterfly arm drills (with no head, body, or leg motion) are extremely productive, but time consuming. These drills are most effective over short distances (10-15 m) at slow speeds. However, the benefit to the swimmers more than makes up for the sacrifice in training time.
In conclusion, make sure a drill reinforces an effective technique element and that the instructions include specific cues to improve swimmer focus.