But when someone says they earn $30K, you probably aren't needing math.  When someone says they paid thirty-seven cents, you probably aren't doing math.

BTCs versus mBTCs will be the same thing.  You'll know what an mBTC is worth and intuitively will know the value when expressed in terms of mBTCs.  Right now, an mBTC is worth about a penny.

- http://en.bitcoin.it/wiki/Units