The bicycle analogy is a good one. I always get confused between low gear and high gear, lets just say the gear that's easier to start peddling but tops out at a low speed vs. the gear that's hard to start but once you get going it's easier to maintain a higher speed.
When applying this analogy to fins, as a spearfisher I find that my movement is always stop and go, I rarely keep up a fast kick for longer than short bursts. This means that I make better use of a fin that's easier to start, and so I've come to prefer softer fins. Coming up from depth they give you the same propulsion as stiffer fins provided you use a correct kicking style. You may find the soft fins to be lacking when swimming against strong current but I use strategy to not have to swim against current. Swimming against strong current is not conducive to good diving even if you have stiff fins because you can't breath up properly. Another area where soft fins may lack is power on demand, such as when kicking hard to keep a fish out of structure. But you can't have your cake and eat it too. If I have to choose between being able to dive comfortably all day without pain in my knees and ankles, or having a small advantage in a rare situation with a big fish but having to cut my day short because of tired ankles, I'll take the soft fins.
You can't compare plastic to the quickness and lightness of carbon. If someone like Dave Mullins chooses to use plastic fins, I believe it has to be to make a statement, because it's hard for me to imagine he wouldn't have access to carbon fins selected specifically for his kicking style. But maybe it is possible if you're only buying off the shelf stuff.
I work closely with my customers to create a fin that is ideal for them. We don't always nail it on the first try, but for sure on the second try. This is usually due to guys wanting a stiffer fin than what they actually need.
Apart from that it can be a complicated choice because overall fin stiffness and flex is a combination of the footpocket and blade. Not all f/p fit everyone well so guys are drawn to the on that does, and insist on making it work. Then you have to select a blade to match the f/p. Take for example the two footpockets discussed in this thread, omer Stingray and Mares Razor.
The Stingray f/p have a soft and comfortable pocket, but the f/p tendons are some of the stiffest I've ever seen. This means even with a very soft blade you're not going to get any flex between where your toes end and the point where the f/p tendons end. I like moderate stiffness tendons in a f/p because they support the blade but also allow it to work. This is not the case with the Stingray f/p. BTW the Stingray pockets will accept a conventional blade back edge, but for it to go completely inside the slot a small recess needs to be shaped in the back edge. See this fin I put together; Stingray f/p with Pursuit MS (medium soft) C90 blade.
The Mares Razor by contrast have very soft tendons. They offer the blade almost no support. This means that to get a moderate stiffness fin you have to combine them with stiffer blade that has most of the action at the tip. It turned out quite well IMO. The Razor pockets are comfortable and the only question is durability and how the lack of support by the tendons will affect the blade in the long run. I think it will be fine.
I use Pursuit C90 med in H.Dessault pockets. 
