I've always preferred and believed that drills with options (or condition games) were the best because they enable you to think, to watch, and to react to your opponent. After all, squash is an open skilled environment as we attempt to hit with great accuracy under pressure while choosing which shot to hit in a fraction of a second. No wonder I have a job! Of course option drills are imperative for players at all levels and I still believe they are the most important. But I believe that basic and repetitive drills are also invaluable for all levels of squash players. Here's why..
The problem I believe with these basic drills for most players (for example, boast drive, drop drive, or rotating drives) is their lack of focus. When doing a repetitive drill like this I always encourage my athletes to have a very specific focus. This might be footwork, shortening their backswing, or preparing their racquet earlier, flattening out their racquet through impact, hitting of their non-dominant leg, adding pace or using height, squaring up their shoulders, etc. The list is endless of things that one can work on and I encourage everyone to select one area as your focus while doing your next basic drill.
An advanced/professional player may have a different focus, for example they may be working on the accuracy of a shot from a certain area off of a specific receiving shot or they may be concentrating on shot disguise or deception, or just the consistency of the accuracy on a shot, whatever the case the same principles for progressive learning holds true.
Once the athlete is able to correctly make this adjustment to their swing or footwork, they should attempt a more advanced repetitive drill. I recommend alternating this shot with another shot allowing for contextual interference to occur (which basically means time to forget what you were doing and having to come back and reproduce the desired action). This will reveal if the change in their technique has sunk in. If it has, then add another shot or two in between the area you were focusing on.
Here's an example of a drill progression I use. If a student and I had been working on their footwork to hit a drive from the front right corner we may start with drop drive and progress to boast drive and then to drive drive boast (so they are driving out of that corner). Or a more advanced sequence would be that the one player can drive or boast, while the player who was focusing on their drives out of the front right corner has to wait until their opponent boasts and then they have to repeat this skill set they were practicing.
Remember: there should be a focus in any drill or condition game. Even if it is just for your 1 warm up drill before you play a match. Don't just do it without consciously trying to focus and improve a part of your game. Practicing smarter is better than harder in my opinion. If you don't know what you 'should' improve, take a lesson or ask a top player at your club. If you ask any of my students they will all probably say either shaping (racquet preparation) or spacing (between you and the ball), so pick one of those as they will likely apply to you as well.