It is the only thing that you can count on providing that all of your measuring equipment is properly calibrated.
I was thinking about that last part yesterday when I transferred one of my beers. To a certain extent, it really doesn't matter if my digital refractometer is off by a little, so long as it's always off by that same amount. So long as you can precisely measure things you can eventually tweak out any accuracy issues as you rebrew and tweak the recipe.
That being said, that digital refractometer was worth every penny. Now that I know that evaporation was all that was throwing my readings off it's been giving the same exact values as my hydrometer, ever single time. Using culture tubes to collect and chill samples eliminates the evaporation while cooling issue, (cooling a sample on a spoon will result in a high reading, due to evaporation.) and makes chilling easy. They're borosilicate glass, so I just run them under cool water for a few seconds. Their volume is so small it takes no time to bring it from boiling to room temperature.