In his blog, Ben Bernanke discusses the merits of longer-term interest rate targeting as a monetary policy tool.

A lot would depend on the credibility of the Fed’s announcement. If investors do not believe that the Fed will be successful at pushing down the two-year rate … they will immediately sell their securities of two years’ maturity or less to the Fed. … the Fed could end up owning most or all of the eligible securities, with uncertain consequences for interest rates overall. On the other hand, if the Fed’s announcement is fully credible, the prices of eligible securities might move immediately to the targeted levels, and the Fed might achieve its objective without acquiring many securities at all.

… A policy of targeting longer-term rates is related to quantitative easing in that both involve buying potentially large quantities of securities. An important difference is that one sets a quantity and the other sets a price. … Concerns about “losing control of the balance sheet” were a factor behind the Fed’s choice of quantitative easing over rate targets while I was chairman.

Conceivably, QE and rate-pegging could be used together … with QE working through reduced risk premiums while the rate peg operates indirectly by affecting the expected path of short-term interest rates. … The principal limitations of rate pegs are similar to those of forward guidance: Both tools are relatively less effective at affecting interest rates at longer maturities, and even at shorter horizons both must be consistent with a credible or “time-consistent policy” path for short-term interest rates.