Eric Hughes, replying to somebody, says:

It takes hours and hours of searching to find a 1024 bit strong prime on a workstation. Granted, you don't need to change very often perhaps, but some people would like to change every day.

If they really want to change that often, they can buy a dedicated machine. There's no good cryptographic reason to change that often, if the modulus is large enough. In addition, changing the modulus can have unpleasant effects on traffic analysis, if not done properly.

For basic PGP mail, there's no need to change keys that often. There are other applications, though, that tend to want new keypairs on each transaction, and looking for strong primes would make them much more annoying, if strong primes matter with the current factoring algorithms... Bill