The average guy on the street has no idea what scientific notation is.
Nor does the average guy know that the calories on your foods' nutrition labels is actually kilocalories. If a pint should ever cost 0.005 BTC, your bartender will ask you for "five mills" or something like that. He won't say "five thousandths of a bitcoin".
But if you're a merchant, you gotta post a price next to your product. I'd rather there were 10 trillion bitcoins in circulation and I could post a price mostly in integers that my customers would understand.
If you ever have to sell something for millibitcoins, other merchants will probably have to do the same and customers will just come to expect it.
Always include units though.