Currently, when you use the materializer, all of the free variables in your input get materialized. When materializing a custom operator, it would be useful to be able to choose which free variables should be materialized and which shouldn't. This would allow, for example, removing the need for several variable cards from intermediate steps to be stored forever, while also allowing an item inventory (or other dynamic variable) in the operator to remain dynamic.

I definitely see the use, but an implementation would (again) not be so straightforward.
Perhaps some time in the future.