Hi paddyb,
It’s not so much a matter of pros and cons as it is that the different layers and components of a mattress serve different functions.
The deeper layers or components of a mattress provide the primary support for the mattress which is the type of support that “stops” the heavier parts of the body such as the hips and pelvis from sinking down too far and putting your spine and joints out of alignment.
The upper layers of a mattress are the ones that “allow” the more protruding parts of the body such as the shoulders and hips to sink into the mattress which redistributes your weight and relieves pressure points and provides “comfort”. They also help to “fill in” and provide support for the inward curves of the body such as the waist or small of the back which is what I call secondary support.
Middle or transition layers help with both comfort/pressure relief and support/alignment to different degrees.
Upper layers provide more of what you “feel” when you first lie on a mattress or go to sleep at night although the deeper layers can certainly affect how you feel over the course of the night and when you wake up in the morning.
There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support/alignment” and “comfort/pressure relief” and “feel” and how they interact together.
Phoenix