Just because they haven't figured out how to back out the pseudorandom variations to your blood pressure doesn't mean it will never happen. And getting these devices into people's hands is the first step in that direction. At some point, we'll be able to take data from continuous blood pressure readings (regardless of arm position) and reference them against time of day, heart rate, activity level, accelerometer readings, diet, etc. And on the other side, we'll have an idea of, not just your blood pressure at rest (which has the virtue of being consistent but isn't the best predictor of heart attack risk), but your blood pressure during daily activity and intense exercise.

Complaining because we're only part of the way there is counterproductive.