Hi rcy1,
You can see my thoughts about the effects of thicker layers or a thicker mattress in post #14 here. More individual layers would also give you more options to customize the “feel” and performance of the mattress either before or after a purchase which can be a benefit for some people that need to do some fine tuning of their mattress.
Yes polyfoam is not as durable as latex and being only 5" it would also compress to a greater percentage of its thickness and tend to be firmer than a thicker mattress that can adapt more easily to different body profiles and sleeping positions but it’s a great option for those who are in a much lower budget range and their return policy also means that the risk of trying it is also low.
Phoenix