For any bond with reasonable trading volumes the ratings are irrelevant, because you can just look at the spread over the relevant benchmark - UST, Bund, JBG, G

For any bond with reasonable trading volumes the ratings are irrelevant, because you can just look at the spread over the relevant benchmark - UST, Bund, JBG, Gilt etc. The market tells you how risky it is; think back to 2010 and the way Greek-German govie spread ballooned out. The problem is with illiquid bonds. Some credits don't trade frequently, so a last trade price may be days or weeks old.