This only applies to browsers. In mail programs you don't want to force the users to deal with their keychains constantly. Also you will potentially have a ton of keys after a couple of years that you have to keep in order to decrypt old mails (if you don't want to reuse the key every year).

Certificate revocation applies to all uses of PKI, not just web browsers. In fact, it is an even bigger problem with email, because there are so many more email addresses than web sites.