A microbitcoin is way too small now and will always be too small. Even after the bitcoin markets arrive at the true value for bitcoins, the micro coin will still be less than 3 US cents. On the other hand a millibitcoin is a larger unit-- one thousandth of today's bitcoin-- and therefore it will settle between 30 US cents and 30 US dollars, which is a much better ballpark for people to deal with that are used to dealing with dollars and cents. These numbers are based on bitcoin world GDP penetration between 0.1% and 10%.

Plus, when it comes to the general public everyone knows what milli means, whereas hardly anyone knows what micro means.