Surely Hectopascals is metric?
Almost, but not quite. It's based on a scale of 10.1325 Pa. The early scientists (including Blaise Pascal, who developed an early barometer) did pretty well in determining the 'standard atmosphere' but got it a bit wrong. No-one has seen fit to change it since.
I think the 'inches of Mercury' approach is just as valid, the big danger is having 2 systems so open to confusion in common use.
TOO