I guess because things sound better in inches and feet?
I just get annoyed because in my engineering course, whilst we use SI units during the learning process we get all these scenarios in other countries that seem to use American equipment measuring in imperial units. Fucking pain in the ass to convert them.
Then you should avoid those. I have a book which has problems based on SI units and there are problems that use imperial units, so i usually try not to solve them to avoid confusion, because as you said converting them is pain in the ass.
Anyway. Today, i was very close in punching someone in the eye at the university.