It becomes pretty useful when the QNH is 992 hPa.. the American crew read back "nine ninety-two" and you get them confirm it's 992 hPa and potentially spotted a 600 feet level bust!!
It happens.. not regularly, but it does. Especially when they come over the Atlantic half asleep at 5am!!
Agreed, the word millibars would have achieved a similar result, but I think hectopascals, being a mouthful, stands out more and it makes it even more obvious that is't not inches of Hg.