As with any other application of cryptography, the keys can become compromised over time. As computers get faster, cryptanalysis becomes more effective. The host possessing the key may be compromised so that the key can be freely obtained. Changing keys periodically can help prevent keys from being compromised. Furthermore, using larger public keys makes cryptanalysis more difficult. However, public keys should not be so large that verification becomes unwieldy (signing is not a part of normal server operation, so it is less critical).
