Whenever Enigmail is installed it should by default encrypt all automatically or manually saved drafts encrypted to the identity's key.
Many a time a user would start typing a message first then go on to setting it for encryption later on. But if the message has already been saved as a draft to the mail provider in cleartext or sent over an unencrypted connect then later encryption is meaningless.
While this functionality could be effected by requiring all new messages to be encrypted by default, this option itself is not enabled by default. Moreover the two functions are semantically different.