From the historical point of view all the forms of new technology that help to pass on more information to the following generations are STRICTLY a positive thing. Just like the emergence of paper, then books and literature,  then photography, then press, then cinema, then television – each added a new crucial dimension of historical data  and helped us, posterior generation, to better understand and visualize our history.  Similarly the emergence of electronic databases, internet, twitter, wikileaks, youtube will help following generations to understand and assess their history and bear tremendous anthropological value.

So I think it is a very bright decision on the part of the US government to archive all public tweets. Likewise, the emergence of wikileaks is a good thing for us in the long run. All  the talks about privacy concerns are a natural reaction of humans to resist additional transparency,  which has always been the case.  And as always the pressure for more transparency will overcome the resistance, and will take its own, as it produces more understanding, more accountability, more integrity, more efficiency, more justice, more happiness for the mankind in the long run.