Primarily, I think this should be done for the benefit of new adopters, the general public, and clueless journalists. Most people aren't used to the basic idea of markets, let alone an explosive one like Bitcoin, so they see the price at $140 and think, 'that's expensive'. When, in fact, the decision to use the unit BTC is completely arbitrary. On the other hand, $0.14 / mBTC looks quite reasonable, so it might help people to make better investment decisions (i.e. buy more Bitcoin).
Firstly, the reasoning for your idea makes no sense because even a child can understand that if a box of chocolate costs £10 that he can get a single bar for less.
Secondly, it's a natural progression as the price goes up smaller units will start to be used.