[…] get every bit of valuable information our of email and into something more permanent. Email is a crappy filing system.
I agree that it’s smart to not use your email programm as an archive, simply because it’s almost always better to keep all files that belong together in one system rather than several. And an email client handles lots of disparate data and a lot of that data belongs to other data on your system. Ideally, you want one system to file and retrieve all your data (even if that’s just the file system).
But I wouldn’t say email is a crappy filing system per se. And it’s as “permanent” as you want it to be. Email is just files. On OS X, every email is a file that’s indexable, searchable and backup-able. There’s really no difference between having an email in Mail.app and exporting that same email as a file. The only difference is that if you export an email, you get to decide where that file goes. And, as alluded to above, that’s an important part of a good filing system.
Server-stored email is a whole other story, though. If you’re using Gmail or something similar as an archive, you better have local backups of everything. Because, as Gabe says, neither Gmail, nor Apple (iCloud) nor any other email provider will guarantee your data will always be accessible. And even if they did, gone is gone. I use IMAP for all my email accounts, but if I want or need to keep an email, I move it to a local archive. I use the server for temporary storage only.