Ok, I agree that you'd convert every unit before calculation, but are you saying that it's just as easy to multiply by 1/12 or 5280 than it is to multiply by 10, 100 or 1000?
For example if I needed to calculate something involving 34mm in SI units, I will often just punch in 0.034 into a calculator without thinking about it. Or 94.5kW I'll just punch in 94500.
If you frequently use imperial units, you may know what the conversion factors are, but you're still going to have to expend some time multiplying out these factors; but if you frequently use metric units, you can just shift decimal points around without doing any calculation.
If you're used to doing imperial unit conversions, then I'm not saying you should switch here and now, but the most important point here is very much related to the original post from +Adafruit Industries
(and here I am playing the "think of the children!" card): if there is this great new metric system in the world that is internationally accepted and endorsed by the global scientific community, easier to learn, less illogical than imperial units, and doesn't require memorising conversions, why then would you teach your kids only imperial units?
Put it in your analogy: if you were a non-English speaker, why would you teach your kids only Latin instead of English? They're both languages sure, but one of them is used internationally, and the other has unnecessary (considered today compared to most modern languages) verb conjugations to memorise, that is only really useful in certain places. (Truth be told, English isn't the most logical of languages either).