The advantage to taper is in the upper unstayed sections. Without the taper (most production beachcats) sheet tension results in more bend occurs in the section close to the hounds than does at the tip. It is easier to design a sail shape to a uniform bend than a bend that isn't. Taper isn't the only way to get a more uniform bend.
Some mfg's taper the layup rather than the section to achieve it. That's not going to happen in alum. though, they always taper the section.

There is no easy answer I can think of to the main point of your question. Sails can be cut to most any mast stiffness within reason. But at both extremes of the spectrum, you become unable to adjust to conditions, and more specialized.