Dash gauges usually fluctuate by a small amount due to variance of voltage etc. The problem is that normal variance of the gauge, amounts to a large percentage of measurable information. So a temp gauge could fluctuate an amount like a 30+ degree change in temps, thus you wouldn't notice that change normally, because it's normal.
I've run a Scan gauge(the original) for years monitoring water temps. Last year I serviced my 98 and ended up with temps running 175ish during the Summer. That's great, I thought I put in a 180* rated T'stat. Then in Winter the temps never got much past 135, while the gauge barely registered at all. When Spring came, the gauge would run up to the normal, near center of left, when the temps got to 160-175. I clearly have a T'stat problem, but the gauge would have made me think something more was wrong, if I hadn't been monitoring the coolant temps constantly. So I knew what was really going on, and it hasn't been a problem. But the gauge wasn't any help at all, and if it over heated, the coarse movement would still be less helpful than a true display of temperature.
