It has nothing to do with cert lifetime, but with how long an authorization is valid to allow you to issue a cert.
If you're using the system properly, the lifetime is irrelevant--it could be reduced to a week with very minimal impact on end users. If you're doing things manually with any regularity, you aren't using the system as it was designed and intended to be used.
Suggest you familiarize yourself with the lengthy and tedious discussion I mentioned above--I have trouble imagining that there are any points that haven't already been made, and discussed ad nauseum. See: