Hi bear12t77,
No it’s not a gimmick at all. It reduces the risk of making an online choice that is not quite the right comfort or support choice and I think it’s a great option to have. It’s not really something that I would use to choose between two materials because this is a matter of design rather than the material and there are also memory foam mattresses with similar exchangeable layers. No matter what the firmness level of a latex mattress though it will still “feel” like latex and memory foam will still “feel” like memory foam but for those who prefer latex (and I am one of them) then choosing a mattress where one or more layers can be either re-arranged or exchanged provides options that can reduce the risk of an online purchase … or sometimes even a local purchase.
As you mentioned, a component mattress also allows you to replace layers that soften or degrade faster than others (typically the upper comfort layers) so you don’t have to replace the whole mattress. They also allow you to accommodate changing needs, circumstances, or bodies over time 
I sleep on my own custom designed latex mattress. You can see the details in post #4 here.
Phoenix