Here's something to consider. What would you do if your entire email archive (work and personal) was blown away overnight? That's becoming a reality for many people at large corporations.1 It's also a real possibility for anyone using email in general. GMail provides no guarantees. iCloud provides no guarantees. Even my own SMTP server does not provide a guarantee. That's the single best reason I can think of to process email as often as possible and to get every bit of valuable information our of email and into something more permanent. Email is a crappy filing system.
Litigation and E-Discovery makes historic email dangerous goods. Perpetual storage also is not very cost effective. ↩