Hi Scotty,
You are bringing up an interesting point.
There are two main functions of a mattress which are pressure relief (mostly called comfort) and spinal alignment (mostly called support). Out of these two … the “comfort” of a mattress is the most obvious and the easiest to test for … for most people. Different people however are more sensitive to different properties of a mattress so if someone has a “bad back” and can immediately feel pain when they are out of alignment … then they may be more sensitive to the support properties of a mattress that are usually more difficult to “feel” when you are testing mattresses. Spinal alignment becomes more obvious over longer term use and “symptoms” of poor alignment can take longer to develop which is why it’s a good idea to have someone help you when you are testing for alignment that can use your perceptions but also use their eyeballs to make sure your alignment is good. Once you have matched a mattress for pressure relief and alignment … then what’s left are the preferences of feel and performance. Some of these (like motion transfer or “springiness and response”) are obvious with testing while some of these (like temperature control) depend on knowing the types and combinations of materials that are likely to do a better job of regulating temperature because this may not become obvious in the short time you are testing a mattress.
In all of these “needs” and “preferences” … each person has a “range” where pressure relief and alignment/support and their other preferences are acceptable or ideal. Those that have a larger range in most of them are the ones that are more on the “I can sleep on anything” end of the scale while those who have a narrow range in one or or more are more on the “princess and the pea” side of the range. All of us are somewhere in this range of tolerance and most people are somewhere in the middle of these two extremes.
So a mattress that has more layers allows for more options to rearrange or exchange layers to be able to adjust or fine tune pressure relief and alignment in all sleeping positions but can also create more complexity for those who are more sensitive and don’t know what to change besides the more common combinations. In many cases … if the top layers are similar … then different types of “fine tuning” can be effective (using mattress protectors, mattress pads, or toppers of various types) because the design itself … even though it is more flexible than most … may not allow for the type of fine tuning that someone is looking for and in some cases the “adjustments” they are making doesn’t quite get them to their goal. For example … if the top layer is 24 ILD in the same thickness in both mattresses … and if their ideal would require 19 ILD … then no matter what they change it may not “feel” quite right. A topper of cheap soft polyfoam may even be an improvement in these cases (although I would normally suggest a better quality material like softer latex or wool etc). In the same way if the change they are looking for is to lower the surface resilience … then additional memory foam, wool, or polyfoam may be a better pathway than changing something in the latex. Part of this depends on what people are used to as well and how strong their preferences are for a certain “feel” that may have nothing to do with pressure relief and alignment.
So without going into the technical details of the effects of various layering combinations and how every layer affects every other layer (and the “putting the layers together” section of the site goes into much more detail here) … it’s enough to say here that for some people … more flexibility of layering and combinations is a value bonus while for others … a 2 layer mattress or even a single layer may be inside their ideal range of needs and preferences and more flexibility may not be worth any extra costs involved. Everything depends on the needs and preferences and sensitivities of each individual.
As a general rule … the deeper layers will have more effect on support and alignment and the upper layers will have more effect on pressure relief and comfort (which are easier to “feel”). Any middle layers will have an effect on both. How much each person feels each of these depends on what they are most sensitive to but for most it’s the comfort which is the most obvious short term factor which is less affected by the deeper layers.
Thicker layers (and mattresses) will also act softer than thinner layers if all else is equal. They also have a wider range or “ride” from soft to firm with increasing compression which means that changes in firmness are more gradual with deeper compression (see post #14 here)
There are certain combinations which I call “dominating layers” were the top layers can be firmer than the lower layers yes. It creates a different combination of surface feel and depth of cradle. For example … firmer latex over softer memory foam can create a more lively and responsive surface with less of a “trapped” feeling but will also allow the memory foam below to show it’s properties (modified by the latex). In the same way a layer of firmer latex over softer latex will lead to “feeling” the firmer latex more (which some people like) but the firmness will be modified by the softer layers below. In both cases … the pressure relief and alignment is what is important no matter what kind of layering is used but these types of combinations can be used to create different feels or to do some fine tuning that may not be possible with more traditional layering where the firmness is more progressive. Everything depends on what the goal of the change or layering is and on the “feel” and performance that someone is trying to achieve. there is no wrong or right … only needs and preferences that can be reached through different combinations of materials.
Hope this helps … but if I’ve missed anything … feel free to post back 
Phoenix