I think George Osborne summed it up best when he said
When George Osborne was opposition Chancellor
// George Osborne has warned that the Bank of England’s strategy of quantitative easing is a “leap in the dark”.
The Shadow Chancellor described the decision to effectively print more money as a “last resort”, necessary because of the “complete failure” of Labour’s other measures to tackle the recession.
He told BBC News, "I don't think anyone should be pleased that we have reached this point. It is an admission of failure and carries considerable risk.” //
http://www.conservati...leap_in_the_dark.aspx
Basically, it is a admission that their policoes have failed. It is a desperate measure.