People indeed can buy as little of a whole coin as they want, but the reality shows that the aim is mostly focused on buying a whole coin. It's all psychologically related where people don't feel satisfied or comfortable with paying big money for a small chunk, while with the same money they can buy plenty of other coins. It's something that can easily be solved by changing the unit nominations of the exchange and wallet clients. Instead of buying 0.10 BTC for $700 you'll be buying 100 mBTC, which is something that will change the way people look at Bitcoin. 1 mBTC will just cost you $7 and I think that people will actually prefer this.