Well, good question. Hadn't thought about that.
I'm assuming that currently, if one were to delete one object (with the minus sign) from a list of five, and then regenerate, the remaining four objects would, by default, be re-scattered, probably with a different random seed and with spacing (depending on the other settings). This might indeed be useful in some circumstances, for example, when deciding whether or not a field of grass really needed flowers scattered on it as well. I think that with the way Skatter works now, this might be the easiest to implement (?).
I was actually thinking of the case where "disabling" one object would leave the other four objects unchanged and where they were. I could see this being useful when deciding between, for example, different choices of species of plants for a hedgerow. You might add three different species to the list, and then quickly toggle between them by enabling/disabling in turn.
But thinking about it, I suspect this may not be easy to implement, because I think the entire scatter is finally made into a single component, making a regeneration necessary each time. And if the regeneration is heavy and takes a long time, then you loose that effect of being able to quickly toggle on and off "layers". Currently, in my example case with five items in the list, I believe a single component results (containing a mix of all five scattered objects). But, using my case of five objects, if five components could be generated, , then it might be easier to simply toggle the visibility of those components. `(In this case, each of the five resulting components only contains one type of object.)
I think I better work with some more complicated Skatter scenes before I can intelligently comment further on my request. I think Skatter already has the capability to do just about anything, so this request can be low priority. I just raise the idea for now.