It's all in the visualization.
An acre foot is used because it is much easier to visualise than a heap of cubic meters. Other traditional measures are also easier to conceptualize than metric and are scaled to human experience.
And yes, the teaspoon measure is highly standardized, so Box's is the same size as mine (the teaspoon, that is).
Metric is easier for engineering calculations, true, but not everything in life is engineering - in fact, believe it or not, most of it isn't (or aren't you getting out much?).
They that have known only metric fear what they do not understand, and seek to impose upon us "the one true measure" - and they get royally ticked off when we chose to use theirs and ours. That alone is enough to make it all worthwhile.
