Perhaps they exacerbate the problem of too much fluoride and who knows what else like aluminum etc. as well.
Research into the relationship between food, central sensitization and fibromyalgia is still in its infancy, although one study (Smith et al) showed that a small group of fibromyalgia sufferers who excluded foods containing MSG, or Mono-Sodium-Glutamat (Natrium Glutamat) and Aspartame from their diet experienced an improvement in symptoms.
CONCLUSIONS: The elimination of Mono-Sodium-Glutamat (Natrium Glutamat) and other excitotoxins from the diets of patients with fibromyalgia offers a benign treatment option that has the potential for dramatic results in a subset of patients.
Given that these two ingredients are rather common in most all processed foods...
I think that as food Science progresses, these synergistic relationships are going to become ever more clear... forget about their harm in singularity... they are much worse when put together.