Hi ps99115,
Outside of the ability to fine tune the comfort or support of a mattress and having more options to rearrange or exchange layers … a mattress that has more layers isn’t inherently better or worse than a mattress that has less layers. If a mattress only has a single layer but is a good “match” for you in terms of comfort, firmness, and PPP then it would be just as good as a mattress that has 4 layers of the same material that was also a good match for you as well.
On the other hand … if a mattress isn’t a good “match” for you then having more layers gives you more options to fine tune the comfort and/or support of the mattress.
I would keep in mind that for most people there is a range of mattresses that would be a good match in terms of comfort, firmness, and PPP but if you are closer to the “princess and the pea” than the “I can sleep on anything” end of the range then the number of mattresses that would be a good “match” for you would be more limited so in this case the ability to fine tune the mattress may become more important.
I would also keep in mind that for most people either the initial combination they choose or perhaps one or two changes in the layering combination would generally have a high rate of success and having more options than just two or three different combinations can sometimes be more complex than it needs to be and can lead to constantly seeking a level of “perfection” based on very narrow criteria that may not exist in “real life” and those people that need more than 2 or 3 options to find a layering combination that works well for them can sometimes end up constantly changing their layering combinations and never being happy with any combination they try.
There is more about the pros and cons of having more layers in post #2 here and there is also more about the effect of thickness (more layers would make a thicker mattress) in post #14 here.
The number of layers and the thickness of each layers are just two of many variables that can affect whether a specific mattress is a good match for any particular person but the only reliable way to know for certain whether a mattress or specific combination of layers would be a suitable choice would be based on your own careful testing or your own personal experience.
It would depend on the condition of the topper, who I was buying it from, how much I trusted them, and on my confidence that the layer was undamaged and in good condition.
These are the types of questions that each person needs to decide for themselves because there are certainly some people that would never be comfortable with sleeping on a layer or on a mattress that someone else has slept on and even the thought of a single hair that belongs to someone else or the thought about what may have “happened” on the layer or mattress that they are sleeping on would be more than enough to significantly bother them and yet for other people it would be “no problem” and the lower cost would be well worth it.
Phoenix