I like to think I'm pretty good at mental arithmetic. Converting feet (0.3) and inches (2.5) are easy enough to do with good accuracy, but lbs requires multiplying the number by 0.45 in your head. Sure I could do that, but it's hardly what I'd call "read the US units no problem", and definitely not something to expect of your general customer base.
The other option would be to try and get a mental "feel" for the size of a foot, 10 foot, what's 5 lbs of water look like, etc. I did something like that back when we switched from Dutch Guilders to the Euro, cause I knew it was going to stick around, so keeping a conversion in my head was just holding me back.
But with US units, nobody's switching, and further, there's no ubiquity in my IRL day-to-day life of things that are supposed to be exactly 1 lbs, 5 lbs, 1 gallon, 10 foot, etc. to calibrate with. The only time I come into contact with inches is in the abstract space of the Internet, where they mean nothing except a quick mental arithmetic puzzle getting in the way of what I actually want to know.
