I suspect that the real reason that they are capturing this information and hanging onto it, is because as various agencies image computers collected in investigations, and collect public and private keys from there, as well as getting public and private keys from businesses such as Google, Yahoo, Bing err Microsoft, et. all, they will be testing the cryptext against each of those keys, and as cryptext is decrypted, they will move that out of the pool of potential evidence, into the pool of material to review to determine if it is material that may be evidence of a crime or national security issue, and handle the material accordingly.
It would not surprise me if some of the suggested botnets that the FBI and others are supposedly running are not capturing public/private keypairs for the very same purpose.
