Good question, Al! Also in response to @too_old.
My reasons are nothing magical. I just aim to resist adaptation through a variety in stimulus, with the intention of better development. Machines having an even resistance curve throughout the movement, free weights being heaviest at the start of a movement, and resistance bands at the end of a movement. Also, you get a greater arsenal of excercises to choose from when shifting from machines to free weights and resistance bands - variety again.
I also have tried the combination of free weights and resistance bands, which can create the most disturbing resistance patterns. For example, the dumbbell squat with a wide/thick resistance band (over the neck/shoulders fixated under the feet) is a killer.
My reasoning is inspired by the thoughts of Brian Johnston who has some interesting theories to offer in his books.
No, I havenāt tried the M3 (Surge challenge) yet. For the time being I canāt subject myself to a fixed program - which is another great benefit of 30-10-30 - You can rely upon getting enough stimulus with only two sessions a week. Even one session can be more productive than just maintenance.
As for different cadence variations on the 30-10-30 theme, I have not tried it specifically, though I liked 30-30-30 for a change (which obviously is another Darden program in its own right). Nowadays, I often mix several Darden techniques depending on which muscles need more or less of something. Applying a bit of freeform, which is another of Brian Johnstonās ideas.
I should probably mention I recently established a heavy powerlifting day a week with a friend, which I combine with a 30-10-30 day a week on my own. Itās too early to tell where this leads, but I strongly believe I could use a better strength foundation for further development. That said, I would not recommend heavy powerlifting to everyone. I just happened to learn a proper technique lately, which I happily investigate/develop. The powerlifters of today are to be considered athletes over a wide array of physical challenges, not too far from crossfit.