I have written on this extensively, but Real Quick:
1. Outside of some military environments, the overwhelming majority of the dynamic critical incidents that outlast the capacity of the firearms being used require slide lock reloads. The magical "lulls" rarely seem to appear.
2. The "tactical" reload (basically as described in the OP), is really "strategic" it is something that is done after a critical incident, in preparation for another one. It is not actually a counter-ambush skill and is much more administrative in nature.
3. It used to be common for square range training to include concepts like "counting rounds" and "double tapping". These antiquated concepts have fallen by the way side in progressive defensive training.
4. In a real incident, if you encounter slide lock you have a much more critical moment than having a "lull" and the opportunity to top off.
When you look at those 4 items, you should get a quick understanding of why we only teach topping the gun off inside of appropriate scenarios. We overwhelmingly stress the slide lock reload in training. As a concept, the "tactical reload" is spot on. As a range habit that gets in the way of actually practicing slide lock reloads (emergency, critical incident, etc), they are a problem. If you are in the middle of engaging a threat and reach slide lock and you've spent years on square ranges counting rounds or otherwise keeping yourself from reaching slide-lock, you won't have an efficient/intuitive response or a well rehearsed technique to get you back into the fight. On the other hand, if you must look down at the gun to do a top-off while behind cover waiting for another threat... so be it. You should spend much more of your time practicing the Critical Incident skill.
The Concept of topping off is what you need to understand, but the mechanics of slide lock reloads is the more vital training issue. If anything, when you are on the square range, you should download your mags to get MORE slide lock training!
-RJP