A. "That'll be 0.00005 BTC please."
B. "That'll be 0.05 mBTC please."
C. "That'll be 50 uBTC please."
D. "That'll be 50 bits please."
A and B are eliminated right off the bat because nobody wants to recall how many zeroes they've put after the decimal.
This leaves us with reasonable C and D, but the word "bit" is far more marketable than "micro-BTC". Micro-BTC just doesn't roll off the tongue like bits does.
I agree that "microbitcoin" is cumbersome to say. However, the natural way of dealing with a long, frequently used word is to shorten it, not replace it. Some examples:
- application -> app
- information -> info
- laboratory -> lab
- maximum -> max
- university -> uni
"That'll be 50 microbitcoins please." -> "That'll be 50 mics please."