I can see adding something like a fiber (silk, wool, etc) to a masterbatched lye solution if a person always uses this type of additive, since the fiber takes awhile to dissolve. After all, the masterbatch concept is around as a way to save time and effort.
But sugar, salt, and similar additives are so easy and quick to add. For the way I soap, I don't see a great savings of time and effort from including them in a master batched lye solution, but I do see a big down side -- I don't want to be limited in my recipe options by the ingredients I include in a masterbatch.
If I add sugar to my masterbatch, and I don't want any extra sugar when making a particular recipe, I'm stuck with taking the time to make a plain lye solution or I put up with a less than desirable situation. To give a concrete example, I make some batches with an infusion of sweetgrass or beer. In either case, the beer or sweetgrass adds plenty of sugars, so I don't want even more sugar added. So a masterbatch lye solution with sugar in it would not be something I'd want to use for this type of recipe.
But if you always use a particular blend of additives or if you're in a higher volume production mode and don't need the flexibility, then try it out and see.
