ECB Purchase of Government Bonds?

Exercised with careful discretion the purchase of government bonds could be a useful practice that might well stabilise markets. Unfortunately, if used immoderately such purchases of weakly performing bonds could ultimately destroy ECB credibility. Such purchases could be considered desperately similar to the ECB's acceptance of Greek junk bonds as security for huge loasns, and as such unecessarily placing the Eurozone and its Institutions at risk.