The only advantage a rotary would have over a flex is extreme cutting power.
I disagree completely. A rotary is tool that is only limited by the user. In contrast with an oscillating machine, a rotary will break down the abrasives at a more even rate, over a uniform area, resulting in a more microscopically perfect finish.
However with a twisted wool pad on the flex using a compound like meguiars M105 there is no paint defect that cannot be removed quickly by a flex. The extra cutting power of a rotary is simply not needed unless you are working on oxidised boats.
Again I disagree completely but I will preface with my experience on the flex is limited. However, on harder paints or paints in terrible condition even a rotary at high RPM, wool pad, and aggressive compound can take a long time to remove the damaged paint. If it takes 5 or more passes of a compound and rotary I could only imagine how much longer it would take by something with far less working action.
A rotary is out gunned in my opinion. There are soft paint jobs that are too difficult to finish with a rotary and a porter cable is needed, this is why people reccomend keeping the Porter cable as they know that Rotary cant do everything. A flex wont have any problem with these soft paint jobs.
On extermely soft paint DA action machines will install micromarring because of the uneven break down of the abrasives. A rotary is required for true perfection of these finishes based because of the more even rate of abrasive fracture (when used by a skilled user). Use a DA on soft Infiniti paint then wipe it with prepsol and you quickly see the damage installed by a DA on soft paint. A rotary will break down the abrasives further then a DA as well, which creates the potenital for a better finish. I should note that I don't have the experience with the Flex, but I would assume that it finishes better then a PC as well, because of the forced rotation.
The flex can remove heavy oxidation from hard single stage paints and can finish hologram free on the softest black paints. A rotary in the hands of your average user could never do both of those.
I agree that in the hands of the average user the flex probably covers enough of a range to produce satisfactory results from most finishes. However in the hands of a skilled user, nothing will compete with a rotary.
I would rather have a flex than both a rotary and Porter cable.
I would rather have a tool that will allow me to take any finish to the highest possible level and that is a rotary. However, like you said, for the average user it may not be the best bet since it requires a lot of skill and learning experience to get maximum results.
Im going to be selling my metabo rotary and random orbital machine for a flex after seeing it in action. My grandma's scratched up honda has extremely soft dark green paint. Even with my average skill levels the flex was 10 times easier to finish hologram free compared to a rotary. On the rotary i would usually have to do an extra ultra fine polishing step to remove the light hologramming. So any saved time is lost IMO. I also didn't have to get the lights into position as i knew the Flex would finish hologram free every time.
A very interesting observation and another I agree with. A DA machine can absolutely finish down "defect" free with more agressive combinations of pads and polishes (you cannot finish a rotary hologram free with an orange pad but I have seen an orange pad finish to acceptable level's on a DA). However, the finish will never be a pure as a finish jeweled with a rotary, so it is always worth the time to (if you are going for the best possible finish) to finish with a rotary, finishing pad, and jeweling polish.
I feel the same way. I'm definitely more at home with the rotary than a DA. Heck, I use the rotary and a white finishing wool to put Desitin on my daughter's bum. No swirls or holograms yet, and she hasn't complained, so I must be doing *something* right.