I agree Laura that until a definitive treatment that consistently shows its ability to reduce progression is found, early diagnosis for the person involved would likely be no more than a source of anxiiety. The only benefit I suppose would be from a research point of view as they may qualify for studies involved in looking at neuronal protection.
One point mentioned in the MJFF Research Paper is: Disease rate biomarker: a marker of disease progression over time (particularly important for developing treatments that can slow or stop a disease). I agree with you: that could potentially help in research studies by evaluating early success of a specific intervention if it has been shown that ultimately the reduction in biomarker levels translates to improved clinical outcome.