Addressing the OP's point about main line weight versus leader weight, I started fishing out of San Diego right about when spectra lines started to become standard usage (2001?). The captains on a lot of boats didn't like the change at first (most people don't like change), but over just a couple of years the idea really took over, and there was no going back. The advantages are obvious.
Like the OP, at first I didn't understand why you might want to fish a lighter weight leader on a stronger (not really "heavier") main line. To understand, you have to change your mindset just a little. The way it was explained to me at the time made the light bulb over my head go on.
The thing is, the fish fighting happens at the end of the line...the hook/knot/leader/leader-to-main line connection. So (I was told), if you're fishing a 40lb fluoro leader, you're fishing 40lbs...even if your main line is 65lb spectra. And if you take a minute to think about it, it makes sense.
It also makes a setup much more versatile, especially when fishing lighter line. Suppose I have Saltiga 40 2-speed spooled with 65lb solid spectra and mounted on a UC 76 Predator (a cobination I use quite a lot). I could use a leader as light as 30lbs, though the rod is a little powerful to fight just 10lbs of drag. But I can also fish that same setup with a 40lb leader, a 50lb leader, and even a 60lb leader...both the reel and the rod will perform amazingly at all of those leader weights. And changing "setups" is as easy as tying a Pena or an FG knot. Meanwhile, the main line cuts through the water easier than even 30lb mono...which, if you think about it, all that the main line really has to do. Under most conditions, I shouldn't have to worry about the main line anyway.
Obviously this is just my own opinion, but it has worked well for me for 20 years, and I envision it working for me for as long as I fish the Pacific out of Point Loma. YMMV.