There's nothing wrong with it per se, or with using metric terms for that matter. I don't even have a strong aversion to either, myself.

But how often do people or cultures apply either to their money? No one walks around talking about kilo-dollars or deci-dollars, or using scientific notation when selling either mansions or bubblegum. It's mainstream for measuring quantities... not for abstract tools like currencies.

The "millibitcoin" term obviously originates from the geekier parts of Bitcoin culture, and is going to face some resistance from non-technical types (read: most people) even outside the USA. Had there already been a name (even a metric shorthand, the equivalent of "cents") for the 0.0001 BTC unit early on, no one would have ever even suggested mBTC, uBTC and the like.