If Bitcoin is going to be mainstream currency one day, which is very likely to happen in few years from now, then even 1 mBTC would be more than enough.

I don't know why, but majority of people interested in Bitcoins are still thinking about Bitcoin only in its basic units (not knowing it can be handled in smaller units), which is one of the biggest issue for wider adoption. Everybody wants to own a coin. Why?! Bitcoin is also mBTC.

21 million of Bitcoins is way too small number in terms of regular people owning at least one whole coin.