Well, the issue is that if you accept that brain state will always be different then there isn't much predictive power in the measure.

The conceit was always that you could measure it across a bunch of people and find the commonly active areas across enough datasets. Even with different baselines, the areas critical to the task would elevate above that baseline.

This paper finds that even in a single person, activity (above the baseline) is poorly correlated across recording sessions. They use a technique called intraclass correlation to measure this: https://en.wikipedia.org/wiki/Intraclass_correlation