Statistics help. They are more important in the essays than in the extended responses, although a casually dropped in statistic in a short response can help flesh out your answer.
For Band 6 and high Band 5 responses you should use some statistics. For example, if you were to be asked an essay on unemployment, knowing the current rate of unemployment would be essential. Knowing the rate of unemployment for particular geographical locations where unemployment was noteably high in the 1990's (i.e. Queensland, Victoria) is not so essential. However, knowing that statistic, provided your description and/or explanation was sufficient, will only improve the quality of your answer.
Statistics tend to have the effect of justifying your answers. For example 'A high Current Account Deficit can lead to a situation known as capital flight, whereby a drop in investor confidence results in large volumes of money being withdrawn from an economy, potentially trigger a major financial crisis. *This effect can be seen in the events leading up to the Asian Financial Crisis of [date] which resulted in [stastistic] la de da*.
It's just a bit of extra knowledge that says to the marker 'Hey Buddy, I know what I'm talking about and I deserve those goddamn marks' etc.
As for a book that doesn't rely heavily on statistics, try Leading Edge economics.
