Hi yngfarm,
The main benefit to having more individual layers is that there are more possible combinations of layers that can be fine tuned by re-arranging them or exchanging them after a purchase. There is more about this in post #2 here. For some people this can be well worth some extra cost and for others that may not have the need for the additional flexibility of more layers it may not be.
Latex cores are usually made in molds that are about 6" thick so you will often find solid 6" layers in many mattresses. 3" layers can be placed on top of each other with different firmness levels (either glued or unglued) to create different response characteristics with the same 6" thickness than you would have with a single layer that was all the same firmness (ILD). One isn’t “better or worse” than the other … they are just different designs that are just one part of how mattresses can be designed differently using different firmness levels and thicknesses of the same material to match the needs and preferences of different people with different body types, sleeping positions, and preferences.
Phoenix