A giant geek friend of mine read my first half of a rough draft of my key booklet. He pointed out a seeming contradiction:
The point of the system (at the most basic level) is to use emotional tension to make the audience invest in a key, then use the key to build emotional tension so you can, you know, use it to get the audience to further invest in a key.
Later, however, I point out that the more the audience is invested in a given key, the more difficult it is to further invest.
The thing is, the returns are quite good for a fairly long time. You're unlikely to "top off" a character even over the course of an entire movie. Concerns for "topping off" are really important only to series, campaigns, and long computer games.
In shorter works, "topping off" isn't really a concern, and the reasons to use multiple character keys is to give breadth of potential situations, rather than avoid inefficient investments.
He also asked for me to put in some theoretical mathy bits. After pointing out that every calculation would be multiplied by the sum over time of two unknown variables, he still wanted it. So I'll probably put in fun mathy bits.