Hi Raildex_, as mentioned on the stream this was a pain point even with material functions as it was impossible to add multiple function calls that happened to have overlapping parameters. The workflow there was to replace parameters as inputs if you wanted them unique, which made the usage a little more tedious. Whilst we’re not quite exposing the control to set each parameter as global/shared yet this is a step towards that. Ideally you’d be able to set each parameter or function call if they are global or owned/instanced as there’s lots of use for both.
For the initial iteration of material layers we took the most common request of instancing parameters for each layer as that opens up the most productivity and workflow improvements by allowing fewer graphs and more variants through layer instances as well as having multiple of the same parent layer per stack, something that’s impossible otherwise and more embraces the goals of the feature.
Ultimately it will be up to you, but we’re not there just yet. 
Dynamic parameter control is vital to many projects and is a core feature of the material system. This is still unfinished for material layers but as mentioned will work for the final release. Changing a material itself at runtime is impossible in the engine as changing the graph requires recompiling the shaders which can easily take several seconds per material. See my reply on page 4 for a little more detail if you’re interested.