EDIT: I'm young and I'm still used to using imperial measurements for alot of things. My Height? 5'10". Distance to school? 13 miles.
But other things, especially small things, I'll use metrics. I'm ~65kg. My laptop is about 30cm across. And I've never really understood pounds/ounces/stone :S
EDIT 2: Wow I'm bad at maths. 210x3 is of course 630 fp.
First question is, why would you measure anything that length in inches to begin with? You would measure it in feet or in feet and inches. There would be no reason to convert. On second thought, you would measure it in miles, or even yards would be fine, but yard as a measurement is rarely used.
I'm the opposite. I can usually get pretty close with my eyes in clicks, but I have no measure of distance in miles, save for extremely long distances (like when I'm planning a trip, I'll use miles. If I'm pointing something out that's a ways away, I'll use kilometers or meters. Maybe decameters if I'm feeling fancy).
One argument against metric that you never hear (because the arguments are always amongst engineers, scientists and shopkeepers rather than people who are interesting) but that is totally valid: Metric measures have shit names.
Imperial measures have interesting names, and they make life a little more interesting when they crop up in idioms and common phrases.
Because when you talk about something that is < 1 foot you don't typically use the phrase "It is 0.16667th of a foot long" instead you would say "It is two inches long."