even if we limit them to Jill Stein and Gary Johnson, forget all the dozens of others who are running, in all honesty runs the risk that those who do not like either Hillary or Donald suddenly see one of the others as a viable alternative. I'm not about to predict who would lose more voters to those two, but this is not a year to set up the possibility of a total cluster-fuck in November.
Most of the voting public would simply not get the nuance of Hillary embracing additional candidates for the debates but Donald objecting. The majority of voters are simply aware there are debates and will (or won't as the case may be) watch them.
It is, of course, highly possible that in a genuine three or four way debate potential Stein or Johnson voters would learn exactly what each of them stands for and therefore be able to make a much more informed decision about voting for them, but I wouldn't count on it.
Plus, it's not Hillary's decision to make about who to include.