You're exactly right in not seeing the advantage.
The most efficient method is to as quickly as reasonably possible get to the effectiveness-vs-side-effects (and counting for costs and availability) level that you want, stay there through as much of the cycle as possible, and then transition rapidly from best gains to best recovery.
The more time spend neither-here-nor-there, with levels not enough for best if any gains but too much to allow natural production or recovery, the more the waste.
The up-and-down dosing methods are a relic of a less informed past (Personal opinion) and have no merit.
As for the three week, actually it can work well provided levels are gotten where they need to be just about immediately, remain where they need to be just about to the end, and then transition rapidly to recovery. This can be done with short acting compounds and orals. However, it's not a popular style.
Could it be worthwhile? Actually I expect yes, if and only if it happened to match up with training program. For example if wanting to do an extremely intensive three week program such as a 3-week Smolov, it could be done, and might be chosen over a longer cycle on the basis that, being so short, it could be repeated again more quickly.
However, I've never actually encountered a situation where I thought 3 weeks was the way to go for an individual person. In principle I think it could fit in some cases, in practice, it sure wouldn't be common for it to be the best fit.