I'm no expert, actually I have no idea what the advantages with any of the curves are. But I'm sure there is method to the madness..

But considering our minimal mechanical advantage over the rig, it seems that having all the bend at the very tip is putting all the action too far away.

This is my visualisation of what the curves look like.. The centre of effort on the sail is not far above the boom..

Left is stiff top, then flex top then crapstant curve..

Barn, do you have to bring Jesus into everything?

Al, are Severne Code Reds any tighter in the leach than Pryde RSRs? They look pretty similar to me. Sure the Reflex is trying to get a more engaged head but is that directly related to mast curve - could they achieve that on a flex top? I haven't seen any Gasstra race sails up close.

In any design choice there are pros and cons. I'm keen to understand the pros and cons of the curve choice. Is it just a mechanism to try and lock you into a brand or is there some science behind it?

The mast will dynamically bend backwards, not just to the side. This backwards bend is just as important to the dynamics of the sail handling IMHO.
The shape of the luff curve pre-tensions the sail. It is designed to complement the bend curve characteristics of the mast so when tensioned/set the sail has the desired shape and dynamic flex.
In race/slalom/speed sails, where a lot of the shape of the sail is determined by seam-shaping, using a different bend curve mast can still make quite a difference to the way the sail sets and handles.
eg: Use a more flex top mast - deeper draught down low, more twist up top for the same down-haul tension. Use a less flex top - Less depth low, tighter upper leech with same down-haul tension. In the first instance the sail will feel more locked and stable and may not pump as nicely. In the second, it will have a great pumping feel but may have less low speed grunt and high speed stability.

I am sure the same applies to non cam sails.

It can be very useful when tuning a speed sail to have the option of trying different characteristic masts. In my case I can also switch tops and bottoms from different sizes as well to change the flex and bend of the mast. It can be a very interesting exercise and can sometime yield great benefits for a particular purpose.

One of the difficulties some of us face is that some race sails are often designed and tested by gorillas that resemble humans. Since I am more like a chimp, I find a softer, or different flexing mast can often make it work for me in a way that closer resembles the intent of the designer.

But as to why some designers start with hard top, and some start with flex top, I am as in the dark as the OP.